Unsealed or damaged grout
It’s tempting to reach for vinegar any time you need a little additional oomph, like when you’re scrubbing the bathroom grout, because vinegar is such a great multitasker. While vinegar does a great job of getting rid of mold and mildew, it can ruin grout if it isn’t properly sealed. Try hydrogen peroxide or oxygen bleach instead, which are less acidic.