"I've seen funny things happen with SD cards in cameras - the first time they are used, an occasional image is corrupt. Sometimes they do it again, even if formatted in the camera... then they inexplicably settle down and are fine!"

I've seen a technical explanation for why this happens, but I'm buggered if I can find it now.
But it's true: there is a period while the card is new when the writing process can be unreliable and it then gets better.