I did some keyword searches and it *seems* like this hasn't been reported before;
When I am spectating as a Mob (e.g. Zombie Pigman, Wither Skelly) and the replay reaches the point at which they die, the game crashes.
Minecraft 1.12.1
Windows 10 x64
Replay 1.12.1-2.0.0-b8
Forge 14.22.0.2461
OptiFine_1.12.1_HD_U_C5
Java 1.8.0_25 64bit (4GB assigned to process)