Let's say you are receiving a verbal report from a fellow from a another country originally, along with a handwritten reference sheet. This guy is from the Philippines. You can never read this fellow's handwriting. You tell him that his handwriting is difficult to read, and jokingly compare it to another co-worker's (well known to everyone) bad handwriting, who is also happens to befrom the Philippines. Then you good humoredly note "you guys are even from the same country".
This previously congenial colleague flies off the handle, extremely offended, and accusing you of racism, calling you a racist. You immediately try to sooth the situation, apologizing, but the co-worker has trouble getting over it. Other co-workers are dumbfounded at the fellows reaction too.