Judging the pool only by clarity
Sparkling water can still be over-stabilised, under-sanitised or corrosively unbalanced. Clarity lags chemistry by days. Insist on seeing actual readings for free chlorine, pH and, in summer, cyanuric acid, rather than accepting a visual all-clear.
Topping up stabilised chlorine tablets indefinitely
Every trichlor tablet adds cyanuric acid that never evaporates. By late summer the pool can be chemically locked, with fine-looking chlorine readings and dull, unsafe water. Dilution with fresh water is the only exit, which is far easier planned in November than forced in August.
Buying a heater or chiller on price alone
An undersized heat pump runs continuously, inflates the electricity bill and still misses the target temperature. Size the unit to your pool volume and desired season, and have the coil cleaned before each winter, because a summer of dust halves its efficiency.