E.g. just yesterday for a short time frame of a few hours maybe half a day or so they had a bug where some closed PRs where shown in the personal which show created _not closed_ PRs.
Or github CI having spurious job cancellations or sometimes on a job failing waits until some (quite long) timeout is reached before reporting it.
Or it temporary being (partial or fully) down for a few hours.
Or it's documentation even through rather complete somehow managing to be often rather inconvenient to use. Oh wait that's not a bug, just subtle bad design, like it's PR overview/filters. Both cases of something which seems right on the first look, but starts being more and more inconvenient the more you use it. A trend I would argue describes GitHub as a whole rather well.