How do surgeons "accidentally" leave stuff in patients?
It’s not as hard as you might think. Guts are dark and squishy and if you put something in them they tend to close around the thing, so stuff gets lost pretty easily. Especially in moments when the surgeon is scrambling to stop a bleeder. This is why counting instruments and needles and gauze in the OR is so important.
There have been plenty of times that I’ve been in the OR (especially C-sections, cuz Lordy they are bloody) where the count came up wrong at the end of a case and everyone had to stop and look for the missing lap. It always turned up though, sometimes on the floor but sometimes hiding in a dark corner of the peritoneal cavity. But before the days of meticulously counting things, it would have been very easy to leave instruments behind.