Serious concern or not? Extraction failure

Went to the range today with my p3at which had close to 500 trouble-free rounds through it prior to this trip.  About 60-70 rounds in, slide went forward and I noticed it didn't go into full battery because the spent shell casing was still partially chambered.  In other words it failed to extract but didn't stovepipe.  I then noticed the extractor was behind the spent round instead of in the groove of the rim like it should have been.  My first though was a broken or loose extractor but it was fine.  I removed the mag and cleared the spent casing, chambered a round and continued shooting.  I put another 50+ rounds through without any issues.
I've been shooting for over 30 years and never seen this.  Is it common?  A sign of serious issues brewing? Or is it a minor ammo related failure.  Was using Independence which I've had good luck with in the past.  Had fired a mixture before getting to the box of Independence.  Round that failed was from about the middle of the mag. :-/

 

Thanks guys! Was kinda thinkin it was nothin to worry about. It was just that most extraction failures I'd seen until then were stovepipes and not an extractor slipping over the rim.
Pretty cool! 1 failure in 500 rounds with a dirty gun.

 

Thanks again. I'm actually quite obsessive about cleaning my guns and I always follow GL's lube guide. It was quite dirty by the time the failure occured from the crud the Independence ammo was putting out. I read somewhere recently that QC issues are starting to show up in a variety of .380 ammo because of the scramble to catch up on production.