The Battle.net Launcher has been in open beta for a while now, so if you're curious to try it, do head over to your account page and pick up the download. The vast majority of the time it works at least fairly well, but like any beta, it has its moments. And one of those moments was this morning when I logged into my WoW install and got this message in the header image. I was a tad confused, as I pay a 6-month rolling sub, but I still freaked out, thinking that my bank account was empty or something. But it was just an error, as confirmed by Blizzard EU Customer Service on Twitter.
@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.