You are unable to place signs on other signs. It just does the placing animation.
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You can place signs on other signs by holfding the SHIFT key. Though this still think that is unintended behaviour.
Makes placing sings on sings in creative an annoying task when you want to place them high. (A place where you need to fly to reach it)

This is caused by the new clickability of the signs. Similar issue for fences etc.

Upgraded to 14w27b, started building in Creative, could not place signs on signs, went back to 14w25b. Yes, too annoying to tolerate in my opinion. Definitely something to be fixed.

Confirmed this morning in 14w28b as well, while I was trying to do a cave spider farm. I wanted to submit it but did not have time to test more snapshots, glad someone else also found it.
Please update the affected version with 14w28b, I don't know and can't test yet on 14w29b.
(@Aäron, when you fly, press shift AND space at the same time, it'll make you able to do shift actions and still stand in position, no up or down)

Kumasasa said "This is caused by the new clickability of the signs. Similar issue for fences etc."
Definition: clickability(Noun) The quality of being clickable, of causing a particular action when clicked
Ok so what action is taken by clicking on a sign? None. Unless they are adding the ability to edit a sign?
I do not have to shift click to place / stack carpet so this seem totally random.

Definition: clickability(Noun) The quality of being clickable, of causing a particular action when clicked
Wiseguy.
http://minecraft.gamepedia.com/Sign#Future
Signs can be created with JSON text which allows complex formatting (colors, bold, italic, etc.), hover and click events,
Is that clickable enough ?

Sorry to be a Wiseguy. I had to think through click vs shift-click and envision other minecraft objects that you have to shift-click to place. I accept that it's consistent, but as Aaron said "Makes placing signs on signs in creative an annoying task when you want to place them high." as is every other shift-click to place task in creative.
In that case the problem is that shift is used for BOTH flying down AND sneaking forcing you to shift+spacebar+click. Those keystrokes should be separately configurable . But that's a different rant :-o