@oliviadgrace Correct, there's a bug causing such messages to popup unnecessarily. But it's also a legit alert for folks who need to top up!- Blizzard CS EU (@BlizzardCSEU_EN) August 29, 2013
So fear not, it is likely a bug. Unless, of course, you really have used up your pre-paid time for that account, in which case get going! It seems from the CS tweet that there may be other random unsolicited popups, so, if you're using the launcher beta, don't take them to heart! All you need to do is hit Okay then log in again.