The last few Fiddler updates, through the auto update feature, resulted in a pop up saying that the update was not properly signed by Telerik. I, of course, immediately cancelled the install.
I ended up going to the Fiddler website to do a full reinstall. This works, but it defeats the purpose of the auto update.
To be clear, the problem here is that Fiddler updated the certificate they were using to sign the builds without updating the AutoUpdater's code that checks the certificate information.
This should have never happened, but certainly should have occurred exactly once-- after you update to a build signed with the new certificate, you should never see this error again.