This is not a bug. In debug mode VS will stop at all exceptions, this exception is handled by a try catch so you just click continue to keep going.
I will say that I did not write the Gravatar.cs, but I just updated the code to have a default value for Size which it did not have before and was thus throwing the exception because it was not in range of expected values if the property was not explicitly specified on the control.
But in general one should not assume that an error that causes the debugger to stop execution is a bug if the error is handled. The debugger will stop on every exception.