For the last year, I've mostly just been maintaining my PAX app by adding the new schedule data and maps for each new PAX event. The last few times I opened it up to do that, I noticed that the design window always showed "Invalid Markup" for my xaml files, and sometimes when I built the project I got a strange avalanche of build errors, including
Error 50 Application is not supported in a Windows Presentation Foundation (WPF) project.
Error 47 Cannot add content to an object of type "ApplicationBar".
Error 5 Cannot add content to an object of type "PhoneApplicationPage".
I had no idea how to fix it, or why it was occurring - obviously, my project was not and never had been a WPF project. I made some notes for myself like 'closed Visual Studio, deleted bin and obj files, re-opened, and it was fixed' - then when it happened again, following the same process wouldn't fix it!
Finally this time, I managed to work it out. These errors occur if I have a *.xaml file open in the editing pane when I run a build. Wow. 100% reproducible, too. At least it's easy to fix...