I'm obviously not a certifiable Visual Studio Guru yet. Ran into a situation many other Dot Net Developers may have, and not even realized it.
I had to revised a couple of Webform pages, moving something from one to the other. Seemingly a simple enough task.
Everything was fine until I did a local publish before submitting for deployment to Stage.
The offending code was jquery and I feared a conflict maybe between it's dollar symbol and that used in the Telerik library, but ruled that out since no other page, all including both libraries, was generating this error.
At that point I started combing through my source codes and that from the browser, and poured through the master file included in the opening content page tag. I found some archaic 'script src' tags in the offending page that I thought perhaps should be in the master file instead. For all I knew, maybe one of my changes to the form structure was causing something to be read or executed in a different order than previous.
So I start shifting tags from content page to master, rebuild and WTF????????
Nothing I added to the master page was in the new content page source code!!!
I finally discovered, upon going through the various master page files, that despite the master page referenced in the content page, a totally different master page was being used. I confirmed this was the case after making tiny revisions in both and seeing the second master was definitely the one included in the build.
In the end, I fixed the jquery error by including a library script that I saw was in the other content page, which also had the same jquery method and wasn't throwing the error. Whatever the reason why my local debug build didn't throw the error, I wasn't about to fight with the Stage build to find out why. I gave it want it apparently wanted, returned the master pages to their original state, error gone.
BUT, I still wanted to know how the hell was the hard coded master page being swapped out and by what setting, or process? I'd already gone back and forth through all the Visual Studio menus I figured might be doing this via some project level setting.
Finally I discovered that Global.asax.cs was overriding the hard coded master with the second one. Learn something new every day. Had never considered that prior. Makes perfect sense if you don't want to risk globally changing the opening tag of every page on your site.