Seagal's Law relates to a wise remark regarding assumptions, made by a character in the Steven Seagal movie Under Siege 2, namely "Assumption is the mother of all f*** ups".
This has been used frequently in the engineering community and is a brief, memorable and useful maxim which leads to sound engineering practices. It is not to be confused with the similarly wise Segal's Law.
References
Seagal's Law Wikipedia(Text) CC BY-SA