Demo desyncing caused by bouncing Lost Souls

In Vanilla Doom, 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 Ultimate Doom, Final Doom and Doom 95 this bug was fixed, causing many Doom II v1.9 demos to go out of sync.

Source

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