While I have never had a spontaneous ejaculation, I do get uncontrolled/unintended erections when wearing speedos.
Last week I went to the pool and had arranged with the Married Guy to maybe hook up afterward in the change rooms. Usually I just wear jeans over my speedos and leave them near the edge of the pool but it was kind of drizzling rain so I went into the change rooms which are a little walk from the pool.
There were a few people in the pool swimming laps, but otherwise no one around. I pulled down my jeans, I was wearing my black ADIDAS 3-stripe speedos under them. I grabbed my goggles and started walking to the pool.
By the time I walked the fifty meters from change rooms to the edge of the pool, I was nintey perfect erect! Just the feeling of wearing nothing but speedos, the feeling of being out in the open wearing very little is such a turn on for me. I didn't waste anytime getting the water and as soon as I started swimming my cock calmed down a little bit.
The Married Guy (you'll have to read my blog to find out how we became fuck buddies) grabbed the lane next to me. After swimming for nearly an hour the Married Guy and I walked back to the change rooms, he had a towel around his waist, I had nothing but my speedos and my growing cock. I'm not sure this erection counts as spontaneous as I knew exactly what we were about to get up to in the change rooms.
The Married Guy and I hadn't fooled around in a while and he had told me before that he was super horny.