In my experience, you can make off by 1 or 2 errors when you accidently include 1 or 2 items in different groups of 5 at the same time. You end up with 25 when

In my experience, you can make off by 1 or 2 errors when you accidently include 1 or 2 items in different groups of 5 at the same time. You end up with 25 when you should have ended up with 23. It may also happen that you don't include 1 or 2 items in the groups, because you thought you'd already included them, while visually scanning the area. When that happens, you end up with 25 when you should have ended with 27.