This is not a big one but it’s good to know.
I was spiking a little thing in a new XNA 4.0 game project and then attempting to include a png into the content folder, the file was there, I could see it (using show all files menu) but when using the context menu “Include in project” nothing was happening. Why?
First I thought maybe the file type is not supported, but PNGs are. Then someone pointed out, is this a project on a Reach Profile? textures larger than 2048 are not supported, this was a large sprite-sheet. So, I changed the profile of the project and I was able to Include the file. 
Happy Monday