Data? Pct?
Here's a case where there was a trial, including multiple appeals, and the vigilantes decided they didn't like the outcome. There is considerable doubt the lynchee was guilty.
http://en.wikipedia.org/wiki/Leo_Frank#Jim_Conley
Also I’d like to point out that once if too often.
People are pretty often wrongly convicted in a trial. Without a trial it is reasonable to assume the chance of error increases.