What causes iPhone show "_D????H?_D?@" message?

[iMore Question]

Today i've opend my iPhone 5S and it showed me a gray screen with a wierd
"_D????H?_D?@" title on it, and an "Okay" button. I didn't tuch it, insted iv'e restarted the device, and it was fine.
Someone knows what causes it?

 

[kch50428]

Random weirdness... if it happens again, and repeats an AppleStore GeniusBar visit may be a good idea.