Type mismatch is a general error that happens when you try to stuff one type of value into a variable of another type.

Let's say you are working with numbers, adding and subtracting them. You're working with

LowVal
HighVal
NewVal

And then at some point you say something like

HighVal = "XYZZY"

your operations are soon going to fail with a type mismatch error.

The easiest way to figure out where you went wrong is to do a response.write of each value along the path. That way you can see exactly what each value is set to, and figure out which one is getting in a "bad" value.