Strange brown gunk in cap?

I just opened a beer I bottled a few weeks ago, and found strange brown pasty stuff in the cap and the inside of the rim of the bottle. None of the other bottles in the batch (or any of my batches) were like this so far.

It didn't smell bad, the beer when poured was perfectly clear but the taste was... different, not bad but different, a little less beer-like than it should be to be honest.

It looked a bit like sediment but the bottle has always been standing upright so i dont get how that could be..

Anyone have any ideas? It's only been open for about 10 minutes so far so, is it still drinkable? I have a real thing about wasting beer... haha

No one has any ideas about this? I'd like to know in case it's something that can affect future bottles/brews.

Did you use DME for priming? I've heard that this can cause a krausen ring in the bottle, but it wouldn't really make sense for only a single bottle to be affected.

Could be an infection, if by "less beer-like" you mean maybe lighter bodied.

Did you use a bleach solution to sanitize the caps? It could simply be rust.

MC