People who aren't trained or professional singers showing off their singing "ability"

Not many things make me cringe more than when some random person starts singing for no reason other than to draw attention to their singing.

The cringe is multiplied exponentially if they close their eyes, plug one ear with their finger, start shaking their head all around and do that false vibrato thing with their mouth.

Nothing says "Hey, look at and hear how awesome I am!" than someone belting out an unprovoked a capella solo.

EDIT For clarification, I am not talking about people who are just having fun or singing along to their favorite music and having a good time.

I'm referring to people who do this solely to draw attention to themselves because they think they're so wonderful that everyone wants to hear them blurt out a solo a capella like they're auditioning for the Tabernacle Choir.