When your passenger manifest grows until the moment the co-pilot slams the door.
When dead bodies don't count! I once did a medi-vac and spent 10 minutes explaining that I'm weight restricted and could only carry 12 people with me. I told the guys to prioritise the casualties and load up. I come back 10 minutes later to see 12 very happy passengers + 3 dead bodies sneakily packed under the seats! I then explained, as diplomatically as possible, that they still weigh something.