Usually what I end up doing is undoing really bad edits by other people: spam, hoaxes, trying to put their own opinion into an article as if anyone else cares, horribly pointless trivia and so forth.
The good thing about Wikipedia is anyone who spots something wrong can change it. The bad thing about Wikipedia is all the people who want things to be wrong (either from bias or cluelessness) can easily do damage that takes multiple good editors five or more times the effort to undo as it took the bad editor to do in the first place.
The eternal struggle
The Wikipedia philosophy can be summed up thusly: "Experts are scum." For some reason people who spend 40 years learning everything they can about, say, the Peloponnesian War -- and indeed, advancing the body of human knowledge -- get all pissy when their contributions are edited away by Randy in Boise who heard somewhere that sword-wielding skeletons were involved. And they get downright irate when asked politely to engage in discourse with Randy until the sword-skeleton theory can be incorporated into the article without passing judgment.
Fun trivia: After I added this section and graphic they have been reused on a number of talk pages and even expanded into the essay Wikipedia:Randy in Boise. It even led to someone getting in trouble after someone who was not paying attention thought that the reference to "Randy in Boise" was outing someone else's secret, real-world identity.
For seeing just how many pages a spammer got to, or how many pages use a ridiculously bad site as a source for information:
Sites that probably need to be removed as references and external links in most cases:
- .findagrave.com (it's annoying how many people put this in)
(I'm sure there are tons I just don't remember right now. I'll add more later. I'll eventually make this a running log so that I go back and check links I previously removed to see if they found their way back again.)
Templates for article improvement
And when there's a bunch on the same article: