Hi,

The arduino serial buffer can overflow if you put breakpoints on or next to your own Serial print commands. If the serial buffer overflows then characters are dropped between the arduino and the pc.

Visual Micro uses a fixes set of serial characters at the start and end of each packet of data outputted by the debugger. If these are the characters that get dropped by the overflow then the serial window will show the data you have described.

If you are using the serial port and find the combination of visual micro debug and your own serial overfloads the serial port then an alternative is to use an FTDI cable with an alternate arduino hardware serial port if available. (see the project properties 'Remote' and 'Local' Port/Transport options)

If no additional hardware serial is available then you can use most digital pins (1 pin is required for trace and 2 for trace/break/modify vars)

I hope this makes sense and that I have understood correctly that the debugger is sharing the main arduino serial port with a program that also uses the Serial port. 

In summary, normally, this problem is avoided by putting breakpoints on code that does not perform serial.print. Otherwise using a 2nd dedicated serial port for debug is recommended.