Or,when initial fermentation slows down,you get a reverse pressure situation. It's called suck back & will suck water from the airlock into the fermenter. That's why I use cheap vodka in mine. Anything that gets in it dies of alcohol poisoning. Water can breed nasties,so you could get an infection. But most likely not,since being past initial fermentation,there's a significant amount of yeasts & alcohol in there to kill or starve them out.