I have done this before quite a few times much to my annoyance. What you've done is mistakenly type a grave accent (`) before entering a number (this can be done very easily if you hit tab a lot like I do). GZDB then interprets this as a string, and names the numeric tag 588 with the erroneous string "`80".
It's nothing to worry about, since the tag still has the numeric value you've set it (588), but it is distracting, I grant that. Haven't figured out a way of renaming tags yet myself so I don't know if you can "correct" this. But if you try it in-game nothing should be amiss.