One of the guys I worked with was at a meeting with clients in Ecuador, they were sitting in the clients office the client's lawyer came in and sat down, as he crossed his leg the gun in his ankle holster slipped out and clanged across the tile floor. It didn't go off, nothing was hurt except for the lawyers pride.
I am not saying this is a reason not to use an ankle holster, obviously the gun and the holster were not well matched or the guy hadn't secured it correctly.