Reserved file names
There are some restrictions when saving your project. Operating systems use some names for their own purpose. Such names are
aux, and some more. The file extension (.ino) does not matter at all. Trying to save your project with a reserved file name will be rejected.
But take care: when you create a tab, in some versions of the Arduino IDE an attempt to save it with an illegal file name will result in a complete loss of your data.