Breaking tiles before detecting the leak
Opening a wet area on instinct routinely destroys tiling that can no longer be matched, turning a leak repair into a bathroom renovation. Pay for acoustic or thermal detection first and insist on seeing the evidence before agreeing where to open. One tile out beats fifteen.
Reusing the old pressure relief valve on a new heater
The relief valve is the safety device on a storage heater, and a scaled, ten-year-old valve on a new tank defeats the point of replacing anything. Insist on a new, correctly rated valve piped to a proper discharge point. A technician who proposes reusing the old one is telling you how the rest of the job will go.
Leaving no paper trail on anything involving water
Water-damage disputes in the UAE are decided by documentation: dated photos, licensed contractors' invoices, detection reports, in-app job records. Cash jobs with no receipt leave you unable to prove the work was done competently — or done at all — when an insurer, landlord or downstairs neighbour starts asking questions.