What's the smallest sound that breaks your concentration completely?

I spent the afternoon in a budget meeting where someone described AI tools as 'incremental cost' and I couldn't stop thinking about how we used to budget for silence itself — three empty minutes between movements. These days the third clarinet seat war is about part-time work while an LLM supposedly covers their admin. So what breaks your focus: the neighbour's fridge hum, a notification, or is it something stranger?