Lonely Moment

Any moment where a seen-by-all verbal or physical gaff leaves you feeling all alone in a crowd

Example 1: When you think you said something super funny but no one else is laughing.

Example 2: taking a tumble in front of a crowd of people but not being injured enough to fake unconsciousness so you're forced to get up, brush off and mumble "darn those stairs someone really ought to fix them" - and think to yourself "crap, that was a lonely moment."