ASP VBScript Fails When Response.buffer = True

I have a big asp file that has an error under certain conditions -- totally
repeatable. However, it only fails when I set response.buffer = True at the
top. WHen I set it False in order to debug it, it works every time! I even
set it to True, but did a .Flush just before the error, and the error won't
happen.

It only happens when response.buffer is True and no .response.flush is issued.

The error is a string variable turns-up empty and crashes a function
requiring a date. I could test for this before the line where it crashes,
but where did my data go? Why is the buffer affecting it?