From memory** what happened was that the I was on a trail that took a fairly gentle left turn, but noticed that the GPS trace had a kink in it. So far, so not very unusual - but then I noticed that the other traces uploaded to OSM had the same left-right kink, and when I went back a couple of weeks (or months**?) later the same think happened.
It wasn’t the sort of multipath confusion that you get with large buildings where a GPS may be all over the place, it was a specific and reproducible offset. Whether modern receivers would show the same issue I’ve no idea.
** it was about 15 years ago.