In Minecraft 1.9 Prerelease 4, entering the Nether leads to a massive memory leak. This manifests as intense lag which starts as unplayable and then continues to get worse, until it's completely locked up, and after killing the process the log contains this error:
[03:32:12] [Server thread/ERROR]: Encountered an unexpected exception
java.lang.OutOfMemoryError: Java heap space
at ass.a(SourceFile:203) ~[1.9-pre4.jar:?]
at ass.a(SourceFile:102) ~[1.9-pre4.jar:?]
at lo.b(SourceFile:147) ~[1.9-pre4.jar:?]
at lo.a(SourceFile:166) ~[1.9-pre4.jar:?]
at lp.a(SourceFile:885) ~[1.9-pre4.jar:?]
at net.minecraft.server.MinecraftServer.a(SourceFile:371) ~[1.9-pre4.jar:?]
at byn.a(SourceFile:213) ~[1.9-pre4.jar:?]
at net.minecraft.server.MinecraftServer.C(SourceFile:571) ~[1.9-pre4.jar:?]
at byn.C(SourceFile:154) ~[1.9-pre4.jar:?]
at net.minecraft.server.MinecraftServer.run(SourceFile:457) [1.9-pre4.jar:?]
at java.lang.Thread.run(Unknown Source) [?:1.8.0_71]
My computer isn't the beefiest machine (a laptop with 8GB of RAM), but never in any previous versions has it even lagged noticeably in the Nether, and it certainly shouldn't be running out of heap memory and crashing just from entering the dimension.
1) Create a new world in 1.9 pre 4.
2) Create a Nether portal.
3) Enter the portal, notice the lag, and wait as the lag gets worse and worse until it ultimately locks up.
(Note: this occurs in at least creative and survival mode; I'm not sure about spectator mode or, probably more relevantly, spectator mode without spectator chunk generation.)