Below is a list of reserved names that cannot be used for folder or file name, either in windows or unix-like file system:
CON, PRN, AUX, CLOCK$, NUL, COM1, COM2, COM3, COM4, COM5, COM6, COM7, COM8, COM9, LPT1, LPT2, LPT3, LPT4, LPT5, LPT6, LPT7, LPT8, and LPT9.
For example, if you try to create a new folder named, "CON" (case-insensitive), windows will "undo" the renaming to the original one. So creating a new folder with the name "CON" just renames it to "New Folder". Try it yourself if you don't believe.
Explanation
This actually is a unix-like feature. DOS device drivers are accessible like normal files, i.e. the everything-is-a-file philosophy. CON is the equivalent of /dev/tty, NUL of /dev/null, COM# of /dev/ttyS#, LPT# of /dev/lp# and CLOCK$ corresponds to /dev/rtc (PRN is an alias to LPT1, AUX is COM1). Every character device can be opened this way, block devices (which are assumed to be FAT formatted...) are named A: to Z:, as you will know. Many pseudo character devices (drivers which had to be loaded as drivers but were no character devices, like EMM386, HIMEM.SYS, ..) had forbidden characters like '*' in their device names to be hidden from the user.
The only problem is: DOS 1, which introduced CON, NUL, PRN and AUX had no directories, i.e. no /dev/, so for compatibility with old DOS1-executables (which don't know that they live in a directory) the character devices have to exist in every directory. Don't know why this feature was not limited to FCB (CP/M like) file access, unix like file numbers were introduced together with directories in DOS 2
Trick
You can however rename/create folders with those reserved names by using the mkdir and ren command in DOS-prompt.
