It's happened to me at least once a week the last month.
I got to check out, ring up an item that had a sale price, it rings up for full price or only a slight discount. Employee says that sale is expired/hasn't started yet, despite being currently tagged.
By the time you get to checkout, your options are to suck it up, go through the entire line and checkout again, or put more labor on their overworked cashiers.
Yesterday, I bought a “$2 watermelon” for $5 for this reason.
It could be due to understaffing or intentional manipulation, but either way it's extremely fucked up.