Minecraft 1.9.1 Pre-Release 2
OS: Windows 7 (ver 6.1, arch amd64)
Java: 1.8.0_31 (by Oracle Corporation)
Launcher: Minecraft Launcher 1.6.61 (bootstrap 5)
Minecraft: 1.9.1-pre2 (updated Thu Mar 10 16:07:24 CET 2016)
The game crashes in a creative test world when many zombie pigmen being agroed towards the player. Possibly due to trying to pathfind to a player that is not reachable. Some small stuttering is noticeable on the mobs, but not as much as in 1.9. The game crashes after about 20 seconds of this.
Edit: Is reproducable, but takes about a minute before it crashes.