"Matt Has A Problem" signaled me with an @ symbol, but I didn't get any kind of notification. Usually, notifications only fail if A. the person has a multi word username and people forget to use quotes, or B. the signal is edited into a post after the fact. There's no sign of an edit, and the link works properly. I haven't touched my notification settings in years either.
It's a relatively minor glitch in the grand scheme of things, but I don't want people to not get signaled, ya know?
Edit: Huh. Just got signaled here at 9:13 CDT... so maybe something weird just happened with the other post. Like I said, very minor.