skip a beat
1. To slow down, pause, or lose one's place, especially when faced with a potential distraction or disruption. Typically used in the negative. The boss didn't skip a beat during the meeting when his son-in-law staggered in, reeking of booze. You can't fluster my mother. No matter how you try to shock or annoy her, she never skips a beat.
2. Of the human heart, to flutter, often from nervousness or excitement. I swear, my heart skipped a beat the first time I saw my wife. My heart skips a beat every time I'm called into my boss's office.