For those trying to set their SafeSearch to locked in Google, they were out of luck yesterday. The key was jammed, the lock was broken and the button would not click.
A Google Web Search Help thread has dozens of complaints and a response from Google about the broken feature.
Google's Nealeigh responded saying:
The team is aware of the issue and is working very hard on a fix for the broken button.
It appears the feature may have been fixed an hour or so ago, as it works for me now.
The Lock SafeSearch feature sets SafeSearch to use strict filtering and restricts the ability of users to change this preference. Strict filtering filters both explicit text and explicit images from your search results. You can learn more about this feature over here.
Forum discussion at Google Web Search Help.