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:
Thanks for mentioning SafeSearch's odd behavior today. Several other people also commented on the same issue. I've passed this post, along with a few others, to our team so they can take a closer look at this.
If anyone has more to add about SafeSearch not sticking, please include information about what kind of computer you have, what browsers are you seeing this problem in, what troubleshooters you've tried, and whether you have any filtering software. It will help me to best figure out what's happening here.
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.