Whenever I ask a question and it's regarded as a statement. Either responding as if it was one, or ignoring it as a muttering.
Every. Damn. Time. At. Work.
It usually takes me up to 5 or more times to ask them before they realize I was asking them a question.
More work-related annoyance: Frivolous complaints about me whenever I stumble or miscalculate a drop onto a counter, and they regard it as me being angry at work because it landed with a *THUD*.
Or simply put: People who look for reasons to complain. Usually, the older they are, the more complaints they look for.
And of course, when I serve people, and I usually follow up with confirming questions regarding their order, they're gone for 5 minutes while I have a line of people waiting for service. Basically leading me to disregard customers if they disappear for longer than a minute, then it's the next person in line to serve; and if they don't respond to confirmations, I default to a usual purchase. Don't like it? You should've been around to make sure I got the order correct. The world doesn't revolve around you. BTW, if the customer is always right; I have many questions I'd like to toss at them.
And excuse my ranting. I tend to get worked up at any mention of my job.
EDIT:
Yes, I do apply the questioning inflection. People are just as tone deaf when listening as they are with singing.