This happens all of the time on 10 meters especially on the low end of the 10M CW range and it's not always caused by hams overlooking checking the frequency before calling CQ.
This past weekend XV1X in Hanoi was blasting in on the extreme low end of 10 CW while at the same time the band was open to South America.
There were 2 SA pirates on AM around 28.000 MHz, almost on top of the XV ham, and if it had not been for me having a 4 element yagi where I could null out the South American pirates, then they would have clobbered Eddy in Hanoi.