XP trap combines Skeleton spawner that drops mobs down from above and Zombie Spawner that transports mobs via water through a lava blade. Mobs had accumulated in the trap before the upgrade from 1.5.0.10 to 1.6.0.1. After the upgrade to 1.6.0.1, some of the mobs (two skeletons, a zombie, and a juvenile zombie villager) began bouncing off edge of the trap where the mobs had accumulated and shooting back up to the head of the water stream. Recording at 30fps (video available upon request), it took the mobs 105 frames to float down the water stream from below the lava blade, and 15 frames to travel back to the top of the water stream. The screen shots show the zombie shooting back up the water stream; they are frames 0 (Still 001), 4 (still 002) and 10 (Still 003) of the quick trip.

Once I cleared out the mob trap, the behavior stopped, and the four mobs fell into the trap as expected.

I have tried, but I have been unable to replicate this issue.