Hi,
offlate, I too have got one or two such errors as "I/O File Error" etc. They have come, irritated me and gone by (generally, when I close and open the same project again, probably the problem is with windows rather than Keil).

I also used to get this strange error (guess it is un documented as I checkedout every page of the material supplied by Keil). The error occurs when I try to use the "printf" function. During Debug, the display switches to some address in the Assembly debugger and the message (something like the below, I dont remember the exact message): "Error accessing Location 0xXXXX... No permission to read/write". This error would come after the printf called the putchar function and the program is waiting for TI to be set. It would simply not get off and I compile it using different Memory models. Then it works sometimes but not always. Once or twice, it so happened that the SAME code, worked with a file in one project but did not work when included in a new project. Sometimes, I feel, the Compiler writers at keil are put to many sleepless nights, and they rarely write a line or two with sleep chasing them. :)

-Cheers,
Harsha.