빌미가 · 빌미를
pretext, cause, opening (for something bad to happen)
Example
작은 실수가 상대 팀에게 공격의 빌미를 주었어요.
A small mistake gave the other team an opening to attack.
Good to know
Typical patterns: 빌미를 주다 (give someone an opening) and 빌미로 삼다 (use as a pretext).