Going to assume this is a bug because it introduces unexpected behaviour. As such:

Bug overview description.
Unprintable (invisible?) characters can be entered where tags are expected, e.g. in tag search.
EDIT: just checked that the link worked and discovered that 1. Firefox replaces precent-encoding in URIs with the non-standard %uXXXX form, and 2. the character in my example is actually 0x200B, "zero width space".

What part(s) of the site page(s) are affected?
Search boxes on /post/index and /tag/index, tag box on /post/show (when editing), possibly elsewhere.

What is the expected behavior?
Tags will have unprintables stripped before processing, meaning <sometag><unprintable> will be equivalent to just <sometag>.

What actual behavior is given instead?
Tags with unprintables exist independently of their doppelgängers.

Can you reproduce the bug every time?
Yes. See this link for examples.

What steps did you take to replicate this bug?
Changing my browser's address bar to bring up posts tagged with spreading%E2%80%8B.