What is the most egregious "interfere with mundanes" crime your PC Magi have done?
Actually, I should split the question in two - done? and got away with?
I take it nobody has ever replaced all the silver coins in the local ruler's war chest, with new coins featuring the face of the guy he is going to war against, where the new coins are possibly of higher silver content?
Does launching an all out war upon a pirate state and destroying a pirate armada count as "interfering with mundanes"?
We sold weapons to both sides of an armed conflict.
But those were explicitly non-magical weapons (of decent quality), so no foul?
Depends. Did your sale cause continued and more intense fighting that expanded what might have been a short and local conflict into something larger that caused a disruption noticeably beyond what would have happened if the forces had to depend on other mundanes to provide the quantity/quality of arms?
(Did you turn a first fight into the American Civil War)
Wait, what? Why?
My players in the Triamore setting made an under-the-table deal with the Duke to provide him with magical assistance in return for him telling the Count of Namur to back off.
Not PCs, but in the saga I'm currently running, an NPC Tytalus has married a young noblewoman and has slain a number of soldiers of their rivals in the next valley.
His marriage is to claim the woman as a vis source. And every fight he's been in, he's been attacked first (except for the first one, where he was threatened with violence).
The rival family happens to be the family of our Flambeaux, who is harassing him for wizardly aid.
In our previous saga, the players were refused entry into Athens at the docks (Because the gift and no social skills, and didn't think about bribes.) They responded by starting a fight and killing six soldiers and escaping into the city to start a manhunt for them. It escalated from there...
Endorsing the murder of a Duchess who was believed to have links to the Infernals by another covenant, with a promise to vote that it wasn't mundane interference if they were ever caught.
It is an interesting topic, most of all as a player. Let's see the list 
First attempt was when the players in the Rhine saga tried to catch and neutralize the Infernalist son of baron Jürgen Schönburg. The guy escaped to the Mauserturm which is extremely dangerous for the novice magi. One of them conjured an illusion with a Tunnel effect: it was a stereotypical black mage threatening the young infernalist with punishment for his failure. Then they collapsed the tower which the guy was in. 
During the collapse they helped the guy, so all the witnesses saw a dangerous black mage threatening the son of the baron and the 3 younglings as the savers of the day.
Second thing was when the characters went to Roznov for the vote of Igor Rastvan. The old brat asked them to help to an old friend near the norther border of the Imperial Lands. They agreed without knowing that the "friend" is Henrik of Mecklenburg, the Black. The task was simple: there is a nobleman with a couple of soldiers. They needed to catch the noble. The playes were eager to roll some dice, so the characters jumped in It turned out that the foreign noble was actually the King of Denmark. They killed a dozen soldiers and a runic wizard. Only Henrik the Black knew the truth, but he sent a chest full of silver to them so they are pretty OK.
Third occasion was when 2 magi went to counter the baltic crusade of the Order of the Sword against the pagan fortress of Muhu. They used all kind of magics against the crusaders. There was an Hermetic Mage on the side of the crusaders, but he was killed in a wizard war before he could make a call in front of the Tribunal. Nasty stuff, high Tremere conspiracy. (The Tremere character is the youngest filius of Stentorius, Exarch of the Rhine and Lotharingia. He didn't let his youngest investment to die because of an overzealous Flambeau coming back from a crusade ) The thing is not a big deal for now. But there is some movement in the Holy See (bc the papal legate was also a participant of the crusade), and a couple of Flambeaus investigating the death of their crusader brother, bc the Tremere character was only half of hermetic age of his enemy in Wizard War. So consequences are highly probable
Fourth thing is that the characters try to secretly interfere with Konrad von Marburg who is the first inquisitor in the history (and in game timeline). They fed him forged clues about infernalism - and the guy killed more than a dozen merchants, ministerials and teachers of the chatedral school in Köln following those clues. He is overzealous and wields serious Divine power, but the characters are mostly pagans, so they think that the guy is just some hedge mage in disguise. A storm is forming, but nothing serious until now.
That's all! We are in the year of 1335. in our Rhine saga. (formed the Lotharinigan Tribunal in 1228.) Best campaign ever!
I switched out the copy of Magna Carta that was due to be signed and inserted one which asserted the independence of Hermetic covenants that had been duly registered. It hasn't got us marched, but did cause some interesting issues at Tribunal.
Magically conjured silver tends to be higher purity than most currency. Assorted MuTe to replicate the coin mold with the new face, and momentarily liquefy the silver bullion and fill the coin molds. Until the right amount of untarnished silver coin, give or take a few coins.
The warchest effectively has not been robbed, so can claim haven't directly interfered with mundanes significantly. But the nobleman is now reluctant to spend them on his campaign, as the bright shiny new coins implies his victim to be is morally/financially/legally in the right, according to some whisper campaigns. And an army not paid is an army not marching.
I realise that all my troupes are boringly law abiding.
The closest we have been was to team up with the King of Munster to arrest some hedge wizards making a ritual to aid an opposing lord from Connacht, where our covenant would otherwise not have been involved.
We heard nothing of it for 17 years, until a new ring of hedge wizards came for a second attempt, and then the magi just refused to help the King, and the Connachtians took over Killaloe. We have just made a truce and a trade agreement with the new Chief, and promised all the hedge wizards involved a wizard's war if they ever pass through «our» valley again.
In case you do not recognise the names, this is in Hibernia with its deviant Peripheral Code.
We did technically interfere with mundanes.
We hunted down a werewolf in Lubeck.
We interfered with a magical tower in Sweden.
We got inside a secret room in an ancient cathedral
Hardly a mundane affair. And you certainly did not take sides among mundanes, which is the real concern.
We did indeed, but the magical tower was not mundane; the offence would be to take sides in the conflict between the locals and the German merchants.
Again not taking sides, so I do not really see the issue here.
Am sure the Church wouldn't agree with your interpretation if they find out, regarding the Cathedral.
The question at hand was not if the Church would approve, but if the Tribunal would.
No, not really.
I'd link you to the saga, but it's not in English, so yeah.
My players bought the island of Malta from Emperor Frederick and ran it as an independent realm. Not “interference” in the Hermetic sense because, as of the end of the campaign, they had not yet brought ruin upon any sodales.
In my campaign, the Magus PC has a Dark Secret and masquerades as his "twin brother" who is a Knight and the Bastard son of the Viscount of Avranches/Earl of Chester. He joined the 1st Crusade under Robert of Normandy. The player regularly interfered with various sieges of cities and various battles, making the 1st Crusade wildly more successful, culminating in the additional capture of Aleppo and the towns all the way down to Damascus. The player is now the "Prince of Damascus", and making war with the Fat Peasant of Bosra.
In addition, the PC pushed the Hermetic Wizards of the Crusade into declaring open warfare on the Order of Solomon.