10 Social Situations Where Everyone Just Ends Up Embarrassing Themselves
7. "You've Got Something On Your Face..."

The usual culprits for this one are either a rogue piece of food or a streak of Biro somewhere on your visage. Unless you consistently are surrounded by reflective objects or mirrors, there's no way to monitor 100% of the time what is on your face - so you may have to rely on others to tell you if there's something. This requires a certain amount of trust with whomever you're with.
The only problem, of course, is that it doesn't always work like that. The person who notices that there is something on your face will most likely be too embarrassed to tell you, for fear of embarrassing you. No, it doesn't make a whole lot of sense from a logical perspective at all.