Quick discovery I made last night.. might save some hair pulling:
In the past I have added tabs using panels (or views) to std node tabs or user tabs… I simply would set their path's to something like node/%node/tab1 or user/$user/tab1
I figured I could do this with completely custom pages as well as long as paths were simliar:
custom/tab1, custom/tab2, etc
It seemed like it should work but it wasn't.
Turns out it sort of was working but there is template.php code required to "enable" tabs for non node/user custom panel pages: