I have seen this type of problem often, usually the server cycle through three different errors, sometimes displaying the correct page. I think what I figured out is that sometimes your script is supposed to end execution, but doesn't, and the adp gets executed anyway, even though the .tcl script were aborted.
For instance (I am running from memory here) ad_maybe_redirect_for_registration, might redirect, but the proc itself returns to the tcl script, so you get two pages returned, more or less. Also, there are similar problems with ad_return_complaint. I think I had to use something like:
ad_return_complaint 1 "Your data is bad"
ad_return_template "blank.adp"
return -code return
in order to get the system to work. This required an empty file called blank.adp in every directory.
