Yes, on several occasions.
I think it could be a number of things - tiredness, a sense of something not being quite right/natural, use of Viagra/Cialis or whichever brand.
For me, the biggest factor is time pressure e.g thinking i'd better get on with it in a 30 minute booking. That's a right negative thought. So, I tend to punt with non- clockwatchers or book an hour, so I can relax.
I was always a slow cummer as a young lad, always took ages, so maybe it's just a biological thing, nothing to do with punting..