One teensy little hole... (spoilers)
When our hero is listening to the radio, the announcer mentions the three robbers... but no mention at all of the hostage that was taken. Of course, this would have given everything away, but still...
Otherwise, everything else that was questionable, like the geographic impossibility of riding rapids that far down a river and winding up back where one started, can be explained away by our hero's state of mind (or lack thereof).
Brilliant performance by Brody in a very good film. Only thing keeping it from being a great film was a somewhat unfocused third act and the hallucination gimmicks that got stale after a while.
I wish there were more movies like it.