Indigestion

"One time he was so sick that he literally threw up a boot."

Gives new hope to the hearers of "...or I'll eat my hat."

*My friend, Michael C, creatively asserts that he hears these phrases all the time.  I imagine he spends a lot of time around pre-school and kindergarten-aged boys.

Ring of Fire

"One time when he peed it burned so bad he was literally peeing out fire."

That's gonna leave a mark!

*Thank you to Michael C for suggesting this idea.