Been tiling for 25 years and have never ever made this mistake. Putting a datum line round a bathroom ready for tiling. Needed to put level line across dooorway. Lined up top of spirit level with level line on one side of the doorway, but for some stupid stupid reason marked the underneath of the level on the other side of doorway. Started tiling, rapid sticky on plasterboard. When tiling eventually picked up, thickness of spirit level out. All it takes is a second to stop thinking and bang, costly mistake.....