I spent the last 30 minutes or so trying to figure out why in the heck my simple Google Maps demo was giving me this error in the console:
The "site" in question was localhost and I knew I was nowhere near my 25,000 daily limit. In fact, the Google API Console reported 44 hits over the past 30 days. To make things even more maddening, when I intentionally included a bad API key I got the exact same error.
Turns out I had forgotten to enable Google Maps V3. This is done in the "Services" menu:
You must turn on API access on the relevant line:
Obvious, right? I just wish the console message had been actually helpful.