Especially with the 5th edition rules, it seems that Parma almost causes more problems than it solves. It certainly brings up many questions. I'd like to pose some of the questions here for starters and assemble (if possible) some of the consensus on it. Here is the first issue with Parma I'd like to bring up (we'll save the 'Magic sword vs Parma' for later ).
First, a magic item that is set up to effect its wearer/holder. The classic Ring of Invisibility, Belt of Strength, Cloak of Protection, or even Cursed Amulet. The wearer puts it on, and activates it (sometimes one and the same). The problem lies in the Penetration and if the user has Resistance. The item has to have a Range of Touch for it work, meaning it needs to affect the wearer, and Parma should resist it. The inconvenience of this cries out for a work around IMO. Some ideas I've had.
The wearer must suppress his Parma, activate the effect, then raise his Parma again. Slightly dangerous, but the easiest way to do this. Still, many magi might want a workaround.
Due to the Parma Ritual's genius, it allows the user to incorporate active magic effects already on the person, such as from an item. So when the user raises his Parma at Sunrise and Sunset he takes into account the items he wears and the Parma ignores them. This relies on interpretation and might not be suited for all players. An option would be to say that the Parma is slightly weaker, 1 lower than normal for example due to exercising this option.
The item must be designed with Penetration in mind. This is potentially costly, but provides a surefire solution, until the wearer's Parma and Arts raise to a level too high for them to be Penetrated by the item. This option should always be exercised for the 'cursed item' magic items.
An option during the invention process would be to add a special modification during the item's creation that would effect the creator only. For +5 (or perhaps more) to the item effect's total level, it would give the item the ability to automatically bypass the creator's Parma and Arts, so its effect always works on him, no matter how high they get.
What are some other options? Do these work? From here maybe we'll go into the more interesting Parma questions.