Thanks for sharing this information. I had the same problem but in a slightly different avtaar.

Had a couple of custom buttons (execute javascript) on some objects which were invoking Apex classes. The java script for these custom buttons was written a couple of months ago and was working fine.

Incidentally, I did not have the any of the REQUIRESCRIPT statements in the java script but it was still working fine for both Administrators and other normal users. All these users were part of a custom profile.

However sometime around last week it stopped working & started showing the error that you mentioned in your post (sforce is not defined). After spending some time, I figured out the problem and after adding the REQUIRESCRIPT statements, my custom buttons are working again.

I’m not sure if there was more to that bug that you described or was a newer release strictly enforcing the REQUIRESCRIPT tag

Thanks
Guneet Sahai