T-Mobile's Web Guard filter is meant to prevent younger customers from browsing adult content on the web — but according to the Open Observatory of Network Interference (OONI), what actually gets blocked can be quite random. T-Mobile says that Web Guard is designed to restrict access to content that falls under 12 different adult categories, which include alcohol, pornography, and suicide. But during OONI's tests, a number of unexpected sites were found to be censored, including a Japanese URL shortening service, a Chinese sports news site, and even the sites for Cosmopolitan magazine, gaming portal Newgrounds, and the Internet Archive: Wayback Machine.
While these results may be curious, T-Mobile does admit that the feature isn't perfect. The carrier hasn't responded to our request for comment, but its Web Guard FAQ explains that the "filter is not error-free and may over-restrict or under-restrict access from the customer's (or parent's) perspective." And so long as you're the primary account holder, you can always remove Web Guard from your account all together.
Update: T-Mobile has provided us with the following statement on the matter:
Web Guard is an optional free add-on feature that allows T-Mobile customers to restrict access to adult-themed (18 or over) websites. Web Guard also restricts access to tools that can disable Web Guard's functionality, such as services that anonymize web traffic. T-Mobile works with a third-party vendor that identifies these websites. Customers have a variety of options for adding and removing Web Guard or for customizing its settings (child, teen, or young adult). For complete details, see the FAQ here.