It's uncommon. pgy5 is very late—latest I can recall seeing is pgy3. Usual reasons are: program A loses a resident, resident from program B applies to fill the spot; or program C disbands, residents scatter and transfer into programs D,E,F, etc.
There's often a pgy mismatch between the resident and the open spot so when a transfer happens it's common for the resident to lose a year or more
I know [of] someone who completed PGY1-4 at one school and transferred afterwards to a different (ranking-wise better) one. IIRC the program A was halted that same year, which I presume was their reason to transfer? Program B had a spot open due to someone dropping out, but it may have been one year off so they had to do an extra research year if I am reading the timelines correctly.