My CCW holsters do not have a retention strap. Holsters not for CCW use do have some sort of retention device, either a lock or a strap, the exception being a Bianchi Askins Avenger holster, which had a removable strap, mine was bought second-hand and the strap was missing.
I remember back in my USPSA competition days, using a TZ-75 Series 88 pistol in a pancake holster with a thumb break strap. It only happened once, but the buzzer sounded, I grabbed the butt of the pistol, my thumb missed the strap release and I gave myself an impressive denim wedgie. Those who observed my misstep were greatly amused. I can see that in a self-defense situation, it would be easy to make the same mistake, but the outcome would not be at all humorous.
I remember back in my USPSA competition days, using a TZ-75 Series 88 pistol in a pancake holster with a thumb break strap. It only happened once, but the buzzer sounded, I grabbed the butt of the pistol, my thumb missed the strap release and I gave myself an impressive denim wedgie. Those who observed my misstep were greatly amused. I can see that in a self-defense situation, it would be easy to make the same mistake, but the outcome would not be at all humorous.