Cracking Your Knuckles Causes Arthritis
"The warning that habitually cracking your knuckles will give you arthritis later in life."
Debunked by one man's decades-long grudge. Dr. Donald Unger cracked the knuckles on his left hand at least twice a day for fifty years, over 36,500 times, deliberately leaving his right hand uncracked as a control, specifically to spite relatives who'd warned him it would cause arthritis. In 1998 he published the results as a letter to the journal Arthritis and Rheumatism: no arthritis in either hand, no measurable difference between them. It won him an Ig Nobel Prize. A larger study of 215 people since has backed up his finding.
Sources: Donald Unger, letter to Arthritis and Rheumatism (1998); Journal of the American Board of Family Medicine study of 215 people.