Upvote:6
It is true that there is no Schengen-wide database of entries and exits, and the different Schengen countries depend on passport stamps to document a traveler's earlier visits to the area.
The good news is that there is no formal requirement in the Schengen rules that hold a traveler responsible for having stamps that match up. You'll need to convince border guards that your plans and history are on the level, and passport stamps are certainly one of the pieces of evidence they look for, but it's not like stamps are judged by a robot that can't consider other evidence.
Depending on how diligent the border guards are the next time you enter, they may or may not notice the discrepancy -- and if they do they will probably be confused but invite you to explain what's going on. You need to be ready to provide this explanation.
If possible, you'll probably want to travel with documentation of your flights to and from Iceland, just in case -- boarding cards are best if you have them on paper, otherwise whatever you can get your hands on. Perhaps the hotel bill for your stay in Iceland?
However, it is unlikely you will need this documentation. Even without it, I would expect that your explanation alone would convince an average border guard. It is corroborated by the fact that the first stamp is from the same date your visa -- in the same passport! -- was valid for, and it would make no sense to assume that you entered Iceland twice without getting stamped and then exited it with a stamp both times.
Furthermore, October 15 is now more than 90 days in the past, so no matter what happened back then, it's not relevant for whether you're complying with the 90/180 day rule now, which is the primary purpose of the entry/exit stamps. (Visas valid for more than 6 months have "90" in the "duration of stay" field, but what that really means is just to follow the 90/180 day rule).