His casual promise turned out to be a banana peel that made him lose trust.

banana peel 字面意思是"香蕉皮"，在英语中常用来比喻"看似无害但实际上会导致麻烦或尴尬的事物"，类似于中文里"绊脚石"的比喻用法。这个表达来源于踩到香蕉皮容易滑倒的生活经验。