Trying to replace tile entities with other tile entities using the /fill command returns java.lang.ClassCastException:
[16:41:25 INFO]: Client> [16:41:25] [Server thread/ERROR]: Couldn't process command
[16:41:25 INFO]: Client> java.lang.ClassCastException: apz cannot be cast to aps
[16:41:25 INFO]: Client> at ali.a(SourceFile:181) ~[14w03b.jar:?]
[16:41:25 INFO]: Client> at aqm.a(SourceFile:484) ~[14w03b.jar:?]
[16:41:25 INFO]: Client> at ahc.d(SourceFile:287) ~[14w03b.jar:?]
[16:41:25 INFO]: Client> at az.b(SourceFile:118) ~[14w03b.jar:?]
[16:41:25 INFO]: Client> at x.a(SourceFile:65) [14w03b.jar:?]
[16:41:25 INFO]: Client> at nr.d(SourceFile:539) [14w03b.jar:?]
[16:41:25 INFO]: Client> at nr.a(SourceFile:526) [14w03b.jar:?]
[16:41:25 INFO]: Client> at ja.a(SourceFile:37) [14w03b.jar:?]
[16:41:25 INFO]: Client> at ja.a(SourceFile:9) [14w03b.jar:?]
[16:41:25 INFO]: Client> at ev.a(SourceFile:163) [14w03b.jar:?]
[16:41:25 INFO]: Client> at nm.c(SourceFile:134) [14w03b.jar:?]
[16:41:25 INFO]: Client> at net.minecraft.server.MinecraftServer.u(SourceFile:545) [14w03b.jar:?]
[16:41:25 INFO]: Client> at net.minecraft.server.MinecraftServer.t(SourceFile:461) [14w03b.jar:?]
[16:41:25 INFO]: Client> at bvz.t(SourceFile:123) [14w03b.jar:?]
[16:41:25 INFO]: Client> at net.minecraft.server.MinecraftServer.run(SourceFile:377) [14w03b.jar:?]
[16:41:25 INFO]: Client> at lt.run(SourceFile:647) [14w03b.jar:?]
[16:41:25 INFO]: Client> [16:41:25] [Client thread/INFO]: [CHAT] An unknown error occurred while attempting to perform this command
Fixed in 14w04a, however, /fill-ing air blocks to tile entities does not work.