Sometimes we'll hit a bug that's so small it hardly seems worth bothering to log it. Maybe it's a missing icon in an uncommon user type in an obscure area of the application. Maybe it's a message that erroneously shows up in a log and is never visible to the customer. Maybe it's something else. In any case, it sure doesn't seem worth the 2 minutes it'll take you to write it up (including a screenshot or logs).
Doesn't matter. Write it up.
Test is not for the lazy. This bug may never become important enough to fix, or it may be really quick and a developer just knocks it out when he's looking for an easy thing to do on a summer Friday at 4:30 pm. But if you don't log it, that bug will always be there.
In the end, one small bug doesn't really matter. When you have several dozen or several hundred small bugs, though, your customers start to lose confidence in your product. It just looks sloppy and adds up to something your customer doesn't trust because it doesn't "feel solid".
No matter how small the bug, it's worth putting it on a path to being fixed.