The code for created bundles cuts a few corners here and there to avoid including unnecessary revlog entries, in particular for the filelogs. When an identical change to a file is made on separate branches, this can sometimes lead to a bundle that, while it can unbundle fine on the repo it was created against, cannot unbundle correctly on an otherwise perfectly compatible repo (i.e. one that has all the bundle's parent revisions. If server.validate is not used, this leads to a corrupt repo (although easily repairable).
Still a thing?
Guess not.