If you accidentally enable IPA input mode or forget to turn it off, its not immediately obvious that anything's different (as most keys are unmodified), but the colon character is replaced by U+02D0 MODIFIER LETTER TRIANGULAR COLON (ː).
This can be pretty confusing to the user, as suddenly when they type titles with namespaces, they don't work anymore (But all other keys are mostly the same). If you're not that familiar with input methods, you probably don't have a lot of hope if figuring out what's wrong on your own. This is bad from a usability perspective.
I'm not sure what would be better usability for this case, but it would be nice if we could make it more obvious what's going on
Quote from commons help request:
In last days, I have a problem with a colon. When I write a colon using keyboard, it is interpreted as a different character (with triangle dots) than the standard colon (with square dots) and doesn't work as a prefix marker. As i noticed, that is not only my problem; see CategoryːEdward Donovan, CategoryːVenom, CategoryːPrague. What is that problem caused by? Formerly I had not this problem. --ŠJů (talk) 18:33, 2 October 2015 (UTC)