P(true) = 0 or p(false) = 1 seem like trivial mistakes to avoid.

Unfortunately they aren't. A universal prior must enumerate all the ways a universe could possibly be. If your prior is based on Turing machines that compute universes, but our actual universe is uncomputable, you're screwed forever no matter what data comes in. Maybe the problem can be solved by a better universal prior, as Nesov suggests elsewhere in the thread, but as far as I understand it's an open problem right now.

ETA: pretty much this whole comment is wrong. The prior is over algorithms that generate sequences of sensory input, not over algorithms that define universes. This is an important distinction, sorry for missing it when I wrote this comment.