Sys.WebForms.PageRequestManager is null or not an object

After merging two ASP.Net projects, one that used Ajax, and one that did not, I began getting a stange error: "Sys.WebForms.PageRequestManager is null or not an object".

After googling around and trying various things, the problem turned out to be an optional tag in the web.config file that I had carried over from the non-Ajax project:

<xhtmlConformance mode="Legacy"/>

Turns out that MS's Ajax doesn't like this. Some folks suggested changing "Legacy" to "Transitional", but when I looked the "xhtmlConformance" tag up on MSDN, it says that "Transitional" is the default, so I just deleted it and the problem went away.

Hope this saves someone a little time.