For some reason, on my iMac,
[alt]+3 will enter the
# symbol as it's supposed to in some applications (e.g. Xcode), while in others (e.g. Safari, Terminal...) it just makes a little error 'beep'.
It's not a major problem, since I can use a different key layout and the problem is resolved, but it's always bugged me that I couldn't determine the source of this behaviour. It has persisted through Mountain Lion, Mavericks, Yosemite.
I also have a Macbook Pro with mostly similar configuration and things installed AFAIK, but it doesn't have this problem.