半路杀出个程咬金/ 半路殺出個程咬金bànlùshāchūgèchéngyǎojīn(also bànlùshāchūgechéngyǎojīn)to have a wrench thrown in the works
literally "Cheng Yaojin ambushes the enemy mid-way"; used to describe someone showing up unexpectedly and disrupting a plan or causing a situation to go wrong