When the piston is activated or disactivated, there is some rendering bug. It works with these blocks: grass, stairs, iron bars, fence and cobble wall
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cannot reproduce, what block is it?

I tested with a grass block
can reporoduce with grass, stairs, iron bars, fence and cobble wall

I updated
(ps: that the hitbox is wrong and that the extension is visible at the back are already reported)

It's one of the problems but I report it for the missing texture.
I think you should remove the other 2 pictures then, as it looks like you are also pointing at that

done
also with fence gate
The texture looks normal in 14w25b but console (both dev and server) throws the following error message when a piston pushes upside down stairs facing north or south:
[14:50:00 INFO]: Client> [14:50:00] [Server thread/ERROR]: Can't setBlockAndData(minecraft:piston_extension, 6) as it's an invalid block state. Defaulting to minecraft:piston_extension[facing=down,type=default] instead (data 0)
[14:50:00 INFO]: Client> java.lang.IllegalArgumentException: Cannot set property ayl{name=facing, clazz=class dt, values=[down, up, north, south, west, east]} to null on block minecraft:piston_extension, it is not an allowed value
[14:50:00 INFO]: Client> at aya.a(SourceFile:131) ~[14w25b.jar:?]
[14:50:00 INFO]: Client> at axs.a(SourceFile:194) ~[14w25b.jar:?]
[14:50:00 INFO]: Client> at ayv.a(SourceFile:618) [14w25b.jar:?]
[14:50:00 INFO]: Client> at alu.d(SourceFile:287) [14w25b.jar:?]
[14:50:00 INFO]: Client> at axn.a(SourceFile:367) [14w25b.jar:?]
[14:50:00 INFO]: Client> at axn.a(SourceFile:154) [14w25b.jar:?]
[14:50:00 INFO]: Client> at ou.a(SourceFile:812) [14w25b.jar:?]
[14:50:00 INFO]: Client> at ou.ah(SourceFile:800) [14w25b.jar:?]
[14:50:00 INFO]: Client> at ou.c(SourceFile:194) [14w25b.jar:?]
[14:50:00 INFO]: Client> at net.minecraft.server.MinecraftServer.w(SourceFile:570) [14w25b.jar:?]
[14:50:00 INFO]: Client> at net.minecraft.server.MinecraftServer.v(SourceFile:506) [14w25b.jar:?]
[14:50:00 INFO]: Client> at coe.v(SourceFile:132) [14w25b.jar:?]
[14:50:00 INFO]: Client> at net.minecraft.server.MinecraftServer.run(SourceFile:422) [14w25b.jar:?]
[14:50:00 INFO]: Client> at java.lang.Thread.run(Unknown Source) [?:1.7.0_45]
[14:50:00 INFO]: Client> [14:50:00] [Client thread/ERROR]: Can't setBlockAndData(minecraft:piston_extension, 6) as it's an invalid block state. Defaulting to minecraft:piston_extension[facing=down,type=default] instead (data 0)
[14:50:00 INFO]: Client> java.lang.IllegalArgumentException: Cannot set property ayl{name=facing, clazz=class dt, values=[down, up, north, south, west, east]} to null on block minecraft:piston_extension, it is not an allowed value
[14:50:00 INFO]: Client> at aya.a(SourceFile:131) ~[14w25b.jar:?]
[14:50:00 INFO]: Client> at axs.a(SourceFile:194) ~[14w25b.jar:?]
[14:50:00 INFO]: Client> at ayv.a(SourceFile:618) [14w25b.jar:?]
[14:50:00 INFO]: Client> at alu.d(SourceFile:287) [14w25b.jar:?]
[14:50:00 INFO]: Client> at axn.a(SourceFile:367) [14w25b.jar:?]
[14:50:00 INFO]: Client> at axn.a(SourceFile:154) [14w25b.jar:?]
[14:50:00 INFO]: Client> at alu.c(SourceFile:2679) [14w25b.jar:?]
[14:50:00 INFO]: Client> at bxv.a(SourceFile:857) [14w25b.jar:?]
[14:50:00 INFO]: Client> at hr.a(SourceFile:45) [14w25b.jar:?]
[14:50:00 INFO]: Client> at hr.a(SourceFile:10) [14w25b.jar:?]
[14:50:00 INFO]: Client> at hd.run(SourceFile:13) [14w25b.jar:?]
[14:50:00 INFO]: Client> at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) [?:1.7.0_45]
[14:50:00 INFO]: Client> at java.util.concurrent.FutureTask.run(Unknown Source) [?:1.7.0_45]
[14:50:00 INFO]: Client> at blj.q(SourceFile:1409) [14w25b.jar:?]
[14:50:00 INFO]: Client> at blj.ap(SourceFile:861) [14w25b.jar:?]
[14:50:00 INFO]: Client> at blj.a(SourceFile:303) [14w25b.jar:?]
[14:50:00 INFO]: Client> at net.minecraft.client.main.Main.main(SourceFile:121) [14w25b.jar:?]
I'm not sure where to post this since the bug is deemed fixed but the error message seems most closely related to this bug. I tested with grass, stairs, iron bars, fence and cobble wall and stairs facing east and west. The only stairs to throw the error are upside down facing north and south.