Whenever I add a property to a custom ASP.NET control, save the file and then recompile the site, the code behinds that I have open will not recognize the new property and the IDE highlights it as an error (which means you also can’t format the code with the Ctrl-K/Ctrl-D chord).
Workaround
Close every file that you have open, then re-open the page you want. At that point, VS will recognize the new properties that were added. If that still doesn’t work, close all the files, recompile, then open the file. If that doesn’t work, restart Visual Studio (and then wait a minute for it to restart).