When placing a sign on a fence, it didn't give me the option to input text, the sign just stayed blank. Placing signs on other signs, grass, and iron bars worked as usual, although I didn't try with anything else.
Linked issues
is duplicated by
Comments

Works fine with everything (even Cobblestone walls), only signs on the side of fences lack the text input menu. That's a weird bug.
confirmed here as well... really odd bug

Interesting detail: Holding the Shift-Key when placing a sign on a fence still opens the text input menu.
Confirmed, and not just in creative mode. Well, that's weird.
Still occurs in 1.6.1pre.
*insert grum's: this feature is work as intended.

What.
I think it was a joke reffering to MC-18365 , where Grum says that a totally weird thing it's a feature.
@Pedro Are you telling me that I'm an inanimate object?
Nope, sorry i'm brazilian, bad english =P
It's ok!
You could use sneak-clicking and still edit them as usual. My bro (nyspace1) found this to and we felt it could go either way as a bug or a feature since you could quickly spam signs.
I was going to post this myself, so that means I can reproduce it. Even thought you can sneak-click it, it should still be fixed.
Yup, what Bob Joe Eisenhower said, oh and both fences, regular and nether brick
If you use shift when placing, it will work! 🙂

Signs are fixed, but there are other side effects, see MC-23384
Confirmed.