People remember the 30 days after and forget the 30 days before. Buying shares, then selling an older lot at a loss within the next month, triggers the wash-sale-rule just as buying back afterwards does.
The disallowed loss is added to the basis of the replacement shares and the holding period of the sold shares carries over, so the deduction is deferred rather than destroyed. The exception is a wash into an IRA, where the loss is permanently lost because basis cannot be added there.
The window spans year end, which is why a December loss harvest followed by a January repurchase pushes the deduction into the next tax year. Dividend reinvestment and automated rebalancing can trigger it accidentally.
This is general information for the United States, not tax advice. Rules change and depend on your circumstances; consult a qualified tax professional.
Related: wash-sale-rule, substantially-identical-securities, capital-loss-carryover, superficial-loss-rule, section-475-election