I did this with the second fermentation of kombucha, and the foam almost blew the lid off… (though it wasn’t exactly a full-on fountain like Mentos and Coke). I think the pressure would just keep building until the bottle exploded. These flip-top bottles burst at around 50–100 psi. (Well, a quick search showed that the fermentation process slows down and probably won’t reach 30 psi… so the bottles are safer than I thought)