When you rightclick on a fence, it clientside behaves as if the fence took the rightclick, and does nothing else, but serverside it places.
This messes up signs, lets you place blocks in yourself, and annoys my ocd.
See: http://youtu.be/Z79WnU7tA9w
EDIT: In 1.6.2 signs are fixed, the other parts aren't though.
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I can assume this is a side-effect of the leash code.

Renamed due to sign part fix in 1.6.2 and ocd not being major. Watch the video.

Duplicate of MC-22395
Duplicate and works as intended. Don't ask me why, maybe it'll be something in the future.
See MC-21433