I've always avoided those carts in the middle of nowhere on the side of an A-road (cos where do they go to the loo and wash their hands), so I've never really taken that much of a risk, however I can relay one noteworthy experience from about 12 or 13 years ago. Me and Mrs Nose had just come out of a gig at the Bristol Bierkeller which finished late, and we were both hungry. The only place still open was one of the chicken and kebab places in the vicinity of the Hippodrome (for those that know Bristol). At the time this place (which was still open when I was last in Bristol last summer) also did pizza, so we shared a large double pepperoni and cheese and ate it whilst walking back to the car. About halfway home Mrs Nose said her tummy was a-gurgling and felt like she needed a shit. And it was downhill from there... We made it home but by then I started feeling sick. We went our separate toilet ways (we had two toilets in that house) and Mrs Nose was basically shitting hot painful water and I was puking. Unfortunately I also then had bum rumbles and had to stop myself from singing into the Shanks pan so I could drop my guts from my arse. Doubly unfortunately I was still feeling sick, so I had to grab the bathroom bin and started puking into that. At one point I was puking and shitting at exactly the same time. In hindsight it was funny, but at the time I really did not feel too clever. Anyway, after the worst had passed me and Mrs Nose were in and out of the toilet - almost wrestling tag-team - for the next few hours until it eventually passed.
Suffice to say the next morning the house needed airing out and we were both physically and mentally exhausted, as well as severely dehydrated. Amazingly, though, there were no accidental sprays anywhere other than the toilets and bin (which had a sturdy liner in it).
We talked about whether or not we should say anything or get in touch with environmental health, but arsed mate, cigs.
But that's the only time I've puke and shit at the same time. And I was probably crying as well.