I wouldn’t dare defile Douglas Adam’s memory by not mentioning that you should keep a towel with you at all times, but my second contender is a surprisingly short three-parter:
- never lie.
- never tell the whole truth.
- never pass up a chance to use a real bathroom.
The XY problem is asking about your attempted solution rather than your actual problem. This leads to enormous amounts of wasted time and energy, both on the part of people asking for help, and on the part of those providing help.