Stock adjustments
Correcting stock, yes. Wrongly, no.

Why it matters
An adjustment changes the book stock by hand: you see some are missing, you correct it. That helps when the finding is right. But an adjustment made too fast, on a quick count or for an error already fixed elsewhere, creates exactly the gap it meant to erase. And since it was done on purpose, nobody thinks to question it.
What often happens
The double correction. A forgotten delivery is logged afterwards, when the stock had already been adjusted to make up for it.
The hasty finding. The item can’t be found on the shelf, so the stock is set to zero. It was in the stockroom.
The adjustment without a reason. Six months later, nobody knows why it was made.
What to do
Limit who adjusts. Few people, and known to all.
Recount before adjusting. Shop floor and stockroom, before touching the stock.
Write down the reason. Each adjustment says why it was made.
Review before the stocktake. Go over the adjustments made since the last one.