I bumped into a fellow binder at a flea market last weekend
It got me thinking how we used to swap tips face to face. Now everything's online, but that chance meeting over some old bindings was a real highlight. He told me about a time he mixed up endpapers and had to redo a whole project last minute.
I did that once, trimmed a whole stack before checking. Totally wrecked the fold. Now I always do the tear test first, right when the paper comes out of the package. Tearing along the grain gives you a straighter line, against it gets ragged. Marking the back with a light pencil arrow saves me every time. It seems like an extra step but it actually saves paper and a huge headache.