While the WebKit project has a strict review process and a substantial amount of regression testing, code reviews can't catch all issues and WebKit doesn't have

While the WebKit project has a strict review process and a substantial amount of regression testing, code reviews can't catch all issues and WebKit doesn't have complete test coverage for all aspects of web technology. Changes can, and often do, introduces regressions from subtle rendering errors to all-out crashes. These sorts of temporary issues aren't that a big deal if it's in a separate application that you use only for testing, but they're a darn sight more painful when you replace your system WebKit frameworks and they start impacting things like your email client or the system installer stack.