It’s a minor thing, but just happened to me: during the Shatterer fight, while in a crystal prison, mesmers can still use Phase Retreat. However, this causes them to get completely stuck in place and unable to use skills (the skills appear normally in the skill bar, but don’t trigger) even after the prison’s been destroyed.
This was fortunately quickly solved by being knocked away by the Shatterer, so it’s not too major. It’s also easily avoidable just by not using Phase Retreat at all while in a prison.