Back in the MS-DOS days, "con" had a special meaning. It referred to the console, and allowed you to treat it like any other file. For example, you might create a new text file by typing copy con new.txt. Then you could enter your text and hit ^Z when finished.
The thing is, you can still do that. Therefore, as far as the file system is concerned there is already an object out there named con. There are other reserved names as well, but I see that while typing this those names have been provided already in other answers.
与恶龙缠斗过久,自身亦成为恶龙；凝视深渊过久,深渊将回以凝视…
