Treat “it’s no big deal” as a cue to take action

The problem may very well not be a big deal; if that’s the case, a calm conversation will take care of it. Similarly, if the person wasn’t being nefarious, simply naming the behavior that’s troubling you in respectful terms should allow the person to address it without a big blow up.

Well-intentioned people who’ve made small mistakes are generally capable of listening without reacting negatively.