Due to a mixup with the documents, my passport was accidentally put into someone else's bag.

mixup 是由动词短语 mix up（混合、弄混）合并而成的名词，mix 意思是"混合"，up 在这里加强语气。mixup 指"混淆、混乱的情况"，特指因混淆导致的错误或混乱状态。