It happens. Especially when there's multiple w2s and 1099s for just one person, being responsible for three people's returns, filing paper forms via snail mail, the actual time of filing happens to be during the stress of being unemployed with a UI glitch so no income despite it being awarded. And later that year separating what remained of the physical forms and shipping all of one pile to one of those three people allowing the chance of one document ending up in the wrong pile. And of course the stress of a natural disaster of biblical proportion happening at the same time as no job and no income.
That year there were 11 tax documents that were thrown on me to properly file while I was trying to get hold of unemployment every single day and trying to figure out how to feed my daughter and the cats. One didn't get scanned. Are you perfect?