Mold in my Apfelwein

I bottled a batch of Apfelwein on 11/20 and I cracked open a 12 oz bottle last night, poured it in a glass and there was a 3/8" diameter mold crusty floating in it! I sterilized everything before bottling and this particular bottle has been in the fridge capped since 11/20. The wine was crystal clear except for the mold chunk. I opened a second bottle and it was fine...

So where could this mold have come from? I didn't stabilize it or anything at bottling just merely siphoned it out of the carboy into sanitized bottles and capped them. The bottles were full to the top within about 1/2" of the cap so there wasn't a ton of head space either... Any suggestions?

I would think it was stuck inside the bottle before it was filled.

I have had mold get stuck to the bottom of bottles even after washing & rinsing. You have to visually inspect each of them before you sanitize them. Kinda gross but it happens.