Painting over moisture instead of fixing it
Blistered or stained walls are a symptom, not the disease. Rolling fresh emulsion over a damp wall near the coast, a leaking AC line or a bathroom with no extraction guarantees the failure returns within months. Find and fix the moisture source first, then prime and paint.
Accepting 'a room' without a definition
Per-room packages are common in the UAE, and 'one room' can mean walls only, or walls plus ceiling, doors, frames and skirting, depending on the vendor. The gap between those two definitions is hours of labour and litres of paint. Pin down exactly which surfaces the package covers before comparing prices.
Skipping primer on gypsum and patched walls
Gypsum board and fresh filler absorb paint at a different rate to the surrounding wall, so unprimed patches telegraph through the final coat as dull, blotchy shadows — brutally visible under cove lighting. A sealer coat costs little and is the difference between a flat, even ceiling and one you notice every night.