Recently after moving one of our managed solution from Dev to Test environment we started getting below Jscript error.
We figured out that error was getting created by this line of code.
The strange thing was that name of the tab was proper in Dev
However, in our Test it got added with the name tab_3 which caused the issue.
The reason I can see for this issue could be that we had multiple forms for that entity. And in one of the forms that tab name was tab_3, however, we weren’t using that form, it was inactive in our dev environment.
Hope it helps..