Okay, here's an odd one:
We played a recent game where my opponent had
Eowyn, Northwoman. His ring-bearer was Frodo. I played
The Balrog, Demon of Might, which says it cannot be assigned to skirmish companions of strength 6 or less. My opponent was able to use Eowyn's text to assign The Balrog to Frodo, even though he had strength 6. Gemp allowed him to complete the skirmish, even though that was not a legal assignment. Eowyn was canceling The Balrog's keywords, but his game text that forbids the assignment should have still been in effect.
Also,
Birthday Present: The card specifies a requirement that the player must choose two events with different card titles. Gemp allowed my opponent to choose only one event (the only one in the discard pile) and retrieve it.
Melilot Brandybuck and
Desperate Defense of the Ring: This was a while back, but I want to say my opponent was able to use Brandybuck to avoid both the burden and the discarding. I don't think this should be possible, since
DDotR presents a choice of two outcomes, which the player must fulfill one of to the furthest extent possible. Unless I'm mistaken, if Brandybuck prevents the burden, then the player should be required to discard the 3 cards instead. This is similar to things like the Boromir ring-bearer playing a card to prevent one of his skirmish triggers, but then being required to satisfy the other one.