Skipping the upturn at wall junctions is one of the most common mistakes — waterproofing needs to run up the wall, not just across the floor, to stop water tracking sideways.
Tiling straight over waterproofing before it has properly cured undermines the membrane before it's had a chance to bond fully.
Floor drains and thresholds are common weak points if the waterproofing isn't dressed properly into the drain body itself.
Balconies are frequently under-waterproofed compared to bathrooms, despite facing direct rain exposure that indoor wet areas never see.