At my age I've had a few. Once I was in for a GI series where they give you that liquid chalk to drink. At first in the waiting room in a gown I noticed the woman with a couple kids glaring at me and I just thought she was having a bad day with her little boy and girl. Several years later it came to me that I was naked under the gown and I had not learned to "girl sit" so they could see everything as I was giving them a great up-skirt.
On that same visit the assistant was the wife of the bartender where I spent my free time. They had me on the table for whatever the machine did and had me roll onto my side. That left my front fully exposed as the gown flopped down off across my back. She tried to pull it up over to cover my front but due to rolling over for the machines view it wasn't long enough to cover my cock and balls so she was forced to study them for the whole test. Every time I met her in the bar after that she always smiled.
In the army I hurt my back and went to the hospital where they decided to put me in a whirlpool for a treatment. My choice was skivvies on or skivvies off. I chose to walk back with dry skivvies, so I'm sitting there naked in the tub waiting for someone to come in to massage my back. Well, in comes this gorgeous blond WAC to do that. As soon as she went hands on my back my cock stood straight up. She could see it slap my belly every time she leaned over my shoulder as her hands slid down my back. She looked at me and smiled then she giggled each time she bent over.
Years later at the VA waiting for an x-ray I met a woman who had been to Nam and was about my age. They called me first to put on a gown in the changing room then in for the x-ray. Back in the changing room I took off the gown when she came in to get ready herself. We continued our earlier conversation as I got dressed and with no hesitation she got undressed and gowned up. The strange thing about that was neither of us seemed to care, we were just two old soldiers changing clothes. Back out in the waiting room she said they shouldn't have full length mirrors in there for us old people to see ourselves in.
Not exactly an oops but my ARNP at the VA mentioned she had a female trainee one day and asked if it was OK with me. I agreed and she had me naked on the table to give me a real thorough exam. She needed help finding my prostate so I got a real fingering from two girls that day. When I had the first appointment with the ARNP she had me bent over the table and as she inserted her finger she said "Oh you have a nice little one". That surprised me a bit as nobody had ever commented on the size of my little wrinkly one before. Then I realized she meant my prostate and I told her I was just thinking the same about her finger. We became friends after that.
My long time family doc used to break up my physical in two visits, top and bottom. For the bottom he did the DRE and I was always naked for the whole 15-20 minute visit. Somehow his B-U-Tiff-L receptionist always had to bring him some paperwork while I was in there.
Then there was the time they sent in a teeny bopper to give me a full body shave before a heart bypass. The gown opened in the back so she did the back first then had me stand and remove it to do the front. She started at my neck then when she got down to my pubes she exclaimed as she noticed I was completely naked; "We need a little modesty" so she put a folded towel across my loins then immediately removed it to shave there. I felt like I was in a Tim Conway skit and had all I could do to keep from laughing out loud.