When a sign is placed on the same block that an item frame is on, the item frame will not output a signal via the comparator on the back.
Turning the item in the item frame still works with the sign on the block but the output will not change.
Once the sign is removed the item frame output works again.
See picture.
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Cannot confirm, regardless if the sign is place after placing the frame or if the frame was placed after the sign, see screenshot.

Kumasasa other sign
Updated:
Sorry for any confusion. Signs only disrupt the signal if on the same face of the block as the item frame. Signs on top do not disrupt it.
Confirmed.
I think this has the same cause as MC-45619.
I can confirm. This happens in both Minecraft 14w04a and 14w04b, regardless of which face of a block I place the Item Frame on (North, East, South or West).
I'm running Windows Vista (64-Bit), using Java Version 7 Update 51.