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.
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.