Well this makes me cringe as i type, but there are two:
1) A good friend from grades 1-6 came up to me outside the tennis courts at my school (match that day against his team) and he said, "Hey, *name*, whats up?" and i couldn't remember his name so i literally just said, "Heyyyyy.... Youuuuu........." and then he didn't even fucking say anything. Just turned and looked away and leaned on a fence. My sister began laughing too, making it worse, and stumbled away. That was probably the worst. Haven't talked to him since.
2) After a tennis match (apparently i just become socially handicapped after tennis), this girl (who i just played) walks up while i was walking with my friends and says hi, apparently in a flirtatious way. I didn't hear and kept walking with my friends, as it was dead silent for like 10 seconds. Then my friend asks why i didnt talk to her and tells me she said hi, and i just cringed and still think about the voluminous amounts of my mom's spaghetti that had been excreted from my pocket on that day.