I made that mistake once.
I was takeing a open water scuba certification class and a bunch of instructers showed up to help out the 2 or 3 regular instructers as the weather was horrible. (wind, waves ,snow flurries)
We were all fighting to get into our gear and this guy who was huge and built had some tats that lead me to believe he was a Marine.Some of us were talking about him as we thought he was out of earshot and I let slip that I thought he might be a "Ex Marine"--in my ignorance even though I ment retired Marine.
He dropped his gear and flew across the beach about 50 ft and stuck his face against mine in the best drill instructer fashion and in no uncertain terms informed me that there are active Marines,retired Marines,dead Marines,but there was no such thing as "ex Marines" because "Once a Marine-Allways a Marine"
Of course it was all for show and entertainment and he turned out to be a great guy who proceded to lead me thru my open water certification.