If you don't do something to limit O2 exposure, you can get some butyrates (vomit smells). Most people put plastic wrap on the surface of the wort, pressing out the bubbles, or flood the vessel with CO2 and seal it up. If it's mild, some of the nasty aroma will drive off with boiling and can age out, but theres really no good way to get rid of it entirely. Another alternative is to pitch some brett and let it age. I believe some strains of brett can convert the butyric acid into pleasant flavor compounds.