Demo desyncing caused by bouncing Lost Souls

Lost souls were originally intended to bounce elastically off floors and ceilings when they hit them, but due to a coding error, the bouncing test was performed after the lost souls' momentum was set to 0. In The Ultimate Doom, Final Doom, Doom 95, and in the released source code, this bug was fixed, causing an incompatibility that makes many Doom II and Doom demos containing the monsters to go out of sync.

Source

 * List of MBF bugs and fixes at the PrBoom page on Sourceforge.net