The Daily UserScript: Userscripts.org Script Versions Tab
A law of usability doesn't exist yet, that I am aware of, but whatever it eventually is, it surely must state that Userscripts.org script pages MUST have a versions tab.
The Unpleasantry:
Take a look at the current process to view the versions of a userscript on userscripts.org:
- Click the "Source" tab.
- Find and click the "Versions" link (if there isn't one then there are no previous versions).
The Alleviation:
Once you install the "Userscripts.org Script Versions Tab" UserScript you have a better option, which is to click the versions tab that now exists, as required by the still undefined law of usability.

@erikvold