The file won't actually show up as having any errors in your package explorer (on the right by default in the IDE), but if you look down in the console (by default down the bottom), you'll see this:
Invalid file name: must contain only [a-z0-9_.]
You might see an error on your project name, but with all the folders and files it can be hard to track down the cause.
When you try to run your application you will see:
'Your project contains error(s) please fix them before running your application'
.. and it won't be happy until you delete the offending file, even if you're not actively referencing it in your code.
Hope that helps.. Happy coding!