Sure. When MH joined OW, there was a glitch where the F would price at Y, which lasted for several weeks. Apparently a reader shared it to lucky and he didn't post until it ended. I think this was fair because AFAIK it didn't showed Y but was ticketed in the fare bucket for F (I think). So in the end BA did upgrade the routings to F and so KUL-LHR was only 35 one way, 70 RT.

It wasn't a "mistake fare" as in it lasted for a long time, so it was more like a glitch. However this AF i'm pretty sure is a mistake fare. Of course maybe this fare only lasted an hour. Unfortunately I didn't realize this until after the glitch got fixed.