In any instance where user-generated content is encouraged, there'll always be a risk that whatever is submitted may be copyrighted material. YouTube is one of the most prominent examples of being a hub where plenty of plagiarized content is submitted simply because users are given free reign over their submissions.
Valve's recently run into this problem with the Steam Workshop, which is a place where the community can submit new content for games that support the Workshop, such as DotA 2 and Portal 2. The community then visits the Workshop, discovers the user-created content, and decides to give its approval (or disapproval.) Generally, this approval/disapproval process manages to flag down materials that violate the Workshop's plagiarism rule.
Unfortunately, the Steam Workshop's strength in the community process is also its flaw. The approval process broke down when a Steam user submitted a mace for DotA 2 from a fantasy MMO called Aion. Rather than flagging the mace down for copyright violation, users overwhelmingly approved it. The item was so popular that it slipped through Valve's filters and became available in the game as a cosmetic item.
Valve caught the problem too late. Plenty of users already had the mace as a part of DotA 2's Sithil Summer Stash. The user who submitted the mace was banned for violating the Workshop's rules and Valve was forced to replace the mace with a different weapon. The developer then put out a blog post pleading for users to abide by the Workshop's rules.
It's a testament of the Steam community that this is really the first issue with copyright on the Workshop since its launch a few months ago. Such is the risk of leaving the entire Workshop process to the community, and it's likely that future incidents will continue to crop up unless Valve changes a few policies.