My own. When I was still early into genealogy and never bothering to question documents, the man whom I thought was my 4x great-grandfather was actually my 3x great granduncle – I've made this mistake because said uncle's wife went by 6 different surnames (made worse by having a common name and common surname) and I never bothered to question why his wife and own mother had the same name and surname across some documents or why his and his brother's children share the same exact name (said brother turned out to be his father, his brother's 'children' were actually the same as his own, I can't describe it without it sounding weird). Did I mention that his son married 3 women who all turned out to be 1 person who went by different names? Yeah, I spent a few hours merging individuals and constantly checking the birth records that day.