tabs work in signature preview, but not in the actual signature.

[Rathmun]

I'm not sure which part of this behavior is a bug. Either tabs are supposed to work in signatures, but don't, or they aren't supposed to, and therefore shouldn't work in the signature preview either.

 

[Facepalmist]

I'm not sure which part of this behavior is a bug. Either tabs are supposed to work in signatures, but don't, or they aren't supposed to, and therefore shouldn't work in the signature preview either.

Click to shrink...

*tests shit*
...Most likely it's the latter.

Signatures are edited in the same format that posts are, which is why you have the Preview option in the first place - as opposed to, for instance, posting a profile comment or making a Report. The reason for this is so that more text formatting options are available when creating / editing your signature - there's a lot of things you can't do in a Profile text or Report, starting with just about everything that ain't plaintext or a bare URL that auto-hyperlinks.

That Preview treats the signature like a standard post (or more specifically, treats it the same as hitting the More Options... button on a standard post), without the native restrictions on Signatures. Those restrictions only come into effect when the edit is actually confirmed.

As for the 'restrictions'? Content tabs are a relatively new feature, so signatures simply aren't coded to support them (or probably a few other things).