Window Z coordinate problems

Mostly just wondering if anyone else has run into this… I am using the select buffer to select rotation points on the screen, but sometimes the results in the window Z coordinates are way off. I rotate a model some, select a point, repeat. Most of the time the point selected is corrected, but some of the time the window Z coordinates get messed up - not completely out there (like unitialized memory), just wrong. Off by 10% here, 25% there, whatever. Everything else works fine! It’s just the window Z!

Anyone else run into this?