If you try to write
[tag] at the beginning of a title, you'll see this error:
But I figured out, writing
[tag] overcomes the error, and the space is stripped on submit anyway.
I don't want this fixed, because it allows me to write a title like: Tag search includes [sass] on search for [*css]
But I'm curious: is it a hidden feature or a bug? It isn't listed at Hidden features of Stack Exchange?
Side note: this error should probably be disabled on Meta sites to allow for titles about tags.