Format ErrorA on an iPhone - What does it mean?

So the other day this pops up on my iPhone 5 and click cancel. Everything goes back to normal. Then I get this again a few more times over the next few days and if I click Accept or Cancel, I just get back to the normal screen and continue as normal. As this is a very benign screen I ignore it.

But then curiosity gets the better of me and I do a Google search. Someone, somewhere must have had this issue, but doing a search for "Format ErrorA" on Google gives absolutely nothing.

I go to the Apple store and show them a screenshot (since I can't reproduce the error on demand) and they seem puzzled as well. They've never seen this. Then they do a bit more research and conclude that they don't know what this is.

Then one of the über-Geniouses at the Apple store says that it reminds him of an error caused by a faulty SIM card and that I should contact my telco to get a new SIM card.

Then all of a sudden, after a few days, these errors stop occurring. Has anyone else seen this? Any idea what it is?