When I was breaking signs I discovered it's hitbox is wrong. BTW, sorry if there's another one of these, i did the search and some is way back in 1.4.4, and some is about rendering so forgive me if it is a duplicate.
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Oh thank god it's not a duplicate. Thanks.

Still happens in 13w39b !
Still happens in 13w39b. Its really annoying.

Confirmed for 13w41a/b !
Confirmed.