To step on a landmine (literally): to trigger an explosive device by stepping on it.
山道には戦時中の地雷が残っていることがあり、踏まないように注意が必要だ。
There can still be wartime landmines on mountain paths, so you need to be careful not to step on them.
爆発の危険がある場所で不用意に歩くと地雷を踏む恐れがある。
Walking carelessly in areas that may contain explosives risks stepping on a landmine.
Figuratively, to touch a sensitive issue or make a serious blunder; to do/say something that causes trouble or awkwardness.
会議であの問題を切り出したら地雷を踏んでしまい、空気が凍った。
When I brought up that issue at the meeting I stepped on a landmine and the atmosphere froze.
彼は昔の失敗について触れられて地雷を踏んだように狼狽した。
He floundered as if he'd stepped on a landmine when someone mentioned his past mistakes.
政治家が不用意な発言をして地雷を踏み、批判を浴びた。
The politician made an ill-considered remark, stepped on a landmine, and was criticized.