What's the worst that could happen when fermenting sauerkraut?
If you follow proper procedures, it's safe. The correct amount of salt (initially) and the acid developed by the desirable bacteria population prevent the process being taken over by non-desirable bacteria. It works. Right around 2% by weight salt to cabbage (or cabbage and...) is a good number, with Stephie's German sources suggesting that possibly even 0.75% would be sufficient. More than 2% is NOT better (I have experimental results, even.) There should be no need to add water.

I personally prefer to use some red cabbage when fermenting in a clear container, as it gives me a visual clue to acidification progress. If using an opaque crock, no point in that (other than if you happen to like pink sauerkraut), as you should not open the crock to check (letting air in is bad practice.) I keep my clear containers in the dark. 

IMHO after a lot of research before I dove in last summer, you should absolutely use an airlocked or watersealed container and leave it alone for 6-8 weeks. The "open crock method" that for some reason seems to be "traditional USA methodology" (why, I can't imagine, we had plenty of immigrants who knew better) is best avoided, as it pretty much ensures a mold battle. Properly tightened (not overtightened, not loose) canning jar lids are one form of airlock (do not loosen them - but keep the jar in a bucket just in case you over-tightened them, as it will contain the mess.) The nutty folks who are checking on it every few days are making something vaguely fermented, but it's not (IMHO) sauerkraut, which takes time; and they raise the odds of mold hugely. "Traditional fermentation crocks" with a water-sealed lid are another approach which works well, albeit generally with a commitment to a larger batch.

I've made 4-5 batches by now, all done under airlock; one (with insufficient weight) did poke cabbage above the liquid line and develop "kahm" yeast - I saw it, I left it alone, I tossed the top layer into the compost when I opened the jar, the rest is fine. With sufficient weight, that should not be a problem. 

So, for the first time ever, I'll plant cabbage next year (I dislike cabbage, but I like sauerkraut.)