Photograph context and detail
Take one photo showing the whole area and closer images of the symptom. Add a familiar object or ruler only when that can be done safely.
Record timing
Note when the symptom first appeared, whether it is constant or intermittent, and what was happening beforehand.
Record system information
Model/serial information, installation date, prior service and warranty paperwork can be useful for appliances and mechanical equipment.
Keep failed-part claims tentative
Write 'water appears below the window after wind-driven rain' rather than 'the flashing is definitely bad.'
Safety boundary. General planning information only. Do not open, dismantle or work on hazardous electrical, gas, structural, fire-protection or contaminated-water systems unless you are appropriately qualified and authorized.