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.
Ignoring the pump room until something screams
A weeping shaft seal, a blocked ventilation grille or a timer reset by a power cut are all cheap catches and expensive failures. The equipment kills more pools than the water does; make sure the visit includes the pump room every time, not just the waterline.
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.