Does anyone have examples?
One of my troupe's mage makes use of 7 League Stride a lot.
I keep thinking that some paranoid mage might expect that. Perhaps an isolated ledge which allows great observation of the paranoid mage's sanctum turns out to be trapped. An illusion over a spiked pit, or a high penetration explosion released when a twig is snapped by human body weight?
Those were mostly for in-Sanctum defence if I recall correctly. I am thinking of traps for those still outside the Sanctum/AotH boundary.
Perhaps more properly I am thinking more of Magi expecting to defend from a Wizard War sometime.
Though it does have good points - rather than a magical trap at the most likely 7 League Stride tactical point, perhaps some hidden bear traps. I mean the 7 League Strider is unlikely to have more than a glimpse of their teleportation destination, so mundane traps should work.
But what of magi making other approaches?
There are two fundamental problems. First making sure that hostile mages would go through the trapped destination, and second making sure only hostile mages went through the target destination. I mean a redcap with boots of the seven league stride are more likely to get caught in your trap than hostile mages in terms of routine usage of a place.
I would have the following problem as a player:
The whole design stinks like some semi-rationalized cheap way from the SG to " counter my power" as a player character.
I would feel as a better option to use Finesse tests to calculate my precision.
EF 6+ vs safe and well-knowned places.
EF 9+ to new or not so well-observable places.
EF 12+ to reliably use it in combat to get behind the back of an enemy.
That would give the option to the SG to see some terrible botch in the end 
I can see that.
But I also see how easy the player character achieved this, so I presume other NPCs do this as well, in numbers great enough that some NPC probably abused it in the past. Hence paranoid magi would consider it, especially if they are on the defending end of Wizard's War. (Am I steering this thread towards Wizard War strategies?)
Sometimes I feel that Magi are too powerful, and are in a sort of genteel nuclear arms race with their amicii.
One mage invents something potentially dangerous, others develop a counter, and then others (for whatever reason) develop a counter to the counter, and so on.
Or perhaps I should really be playing a Tytalus or Tremere mage (one doing it for the challenge, the other playing chess gambits). I think I have some confusion about why I started this thread - this helped.
It all comes down to finding a fine balance between giving a good challenge for the player and simply saying "No!", because as ST we were not ready to handle the situation.
Usually, the way I handled it is to let it work the first time, because they deserve the credit of finding an interesting way to circumvent the challenge I set in front of them. Even if it means trashing a good chunk of the story I planned since their solution bypass it.
And I will congratulate them for it. But I will also remind them that next time won't be so easy - their next target would have considered this eventuality. It gives them a sense of achievement as well as some teaser for the challenge to come.
I also apply the principle of symmetry: once PCs start doing it, NPCs will also have access to this "trick". I always start designing NPCs and protagonists using only core powers/spells, then slowly turn up the challenge by designing more unique powers/spells to give a good, but not insurmountable, challenge.
In a nutshell, like a new product launch in a market: the first mover has the initiative and reap the reward, but the competition will quickly catch up.
Yeah, I know this feeling about 'arms race'. As a SG I try to paint a picture about the Order that they have some presumtions and "customs".
It is like being a brute to have only (or too much) combat focused magics. Those are for hoplites. Think about professional boxers: they can hit you really hard, but in most of real life situations this particular skillset is not very useful outside of the ring.
In fact, if he hits you, he'll be in trouble. Bc law enforcment and mainly bc of the reputation impact that the offender is an uncultured, Code-breaking brute. Other, more experienced mages talk about this things with their sodales or filius/filia. The Order is a culture, not just a group of people with powers.
The Order has its history about the arms race and there are "institution" to stop this waste of resources. One system is the amicus system. Other is that good researchers are sponsored by House Tremere or other powerful magi. They will protect those not so well-armed sodales. More experienced magi should explain this to the young ones that no magi is alone: everybody has friends, parens or sponsores. And there are the Quesitores: they have the incentive to heavily punish the those Code-breakers who would force other magi into an arms race. That is a well-knowned problem among the quesiores that they are an institution not just for justice, but also for prior deterrence.
I have no problem with shenanigan PCs IF they have a clever idea, and they put effort, seasons and magical resources to think about how to solve a situation when they break the code. They should know that the Code can be broken, but you should be really clever and well-prepared to get away with the case.
A paranoid mage might try to create a regio, which is covered in Realms of Power: Magic. Some of them are more trap-like than others.
Keep in mind that "other magi have abused this" does not necessarily mean "this is a threat to the magi not abusing it" seven league stride is largely a way to move around quickly, not deliver strategic strikes and penetrate an aegis. The only necessary defense is to keep your covenant's interior concealed.
There is also the problem when players come from games where it is no holds barred combat.
If the code was strictly enforced, then most of them would have gotten Wizard's Marched within a few sessions. Rather than dictate their actions completely, need some wiggle room in "historical precedent" to let them off with warning or similar reasonable doubt.
I'd say Ars Majica is the no holds barred combat.
Certain other games, regular combat is expected. One doesn't use all one's resources in a combat. There's no need for the alpha strike. A poor decision just means a bad turn most of the time.
Ars Majica, players make poor decision, or are just unlucky, characters die. Each round is a chance of a 1 being rolled and debilitating or killing someone. An Ars Majica character choosing to fight is not a decision taken lightly, and they want the fight to end very quickly, using all the resources at their disposal.
Agreed. Those Tremere magi broke the code extravagantly and if there was any justice in the world, they would have been marched. I don't know of anything the PCs have done march worthy.
To be more of a forum post, than 2 people in the same game chatting on a forum.... I'd like to think most players quickly understand they aren't loot lusting murder hobos. Having a home base, where any enemies you make can find you, the relatives of those you kill can find you, really locks in there are consequences for recklessness quickly.
On a similar vein than this topic, One could have fires (either mundane or magical) and a trap that fills the place with hay. Mundane fire, magical fuel, anyone?