Google has a search preference that allows a searcher to define how strict the adult filter should be. It is called Safe Search and I think it was originally designed by Matt Cutts.

In any event, I am seeing dozens of reports of the Safe Search filter getting “stuck.” The largest standalone thread is at Google Web Search forums which describes the issue as users not being able to remove the safe search preference to something more lax.

Google Employee Skylar confirmed the issue in the thread and said they are looking into a fix. Skylar said:

At about 2:30am (EST), this morning, Google seemed to have fixed the issue. I am not sure what caused it, because Google has not yet confirmed the fix, but we have at least two reports saying that issue is now resolved.

Forum discussion at Google Web Search.