...so, I have this really nice solid wood door with a mirror on both side I got custom specs for our space. Went to paint it and in retrospect my method for wanting to achieve nice paint lines didnt work. I had slide some thin paper between the wood and glass, and well, paint seeped under it and bled in between the wood and mirror. lesson learned.

The image you see shows the mirror reflection on the right, you can see where the now blue wood panel is on the left. That white blotch is the backside of the wood panel where the paint didn't get to. You can now see this white splotch when looking at the mirror. What is interesting is - the reflection seems to be coming from the backside of the mirror, so even if the gaps fills, the reflection angle is peeking further underneath...hard to explain.

I still have the other side to do (front side), so if done right it will look nice...how to go about this?

To correct this side, my thoughts were to just jam the gap full of paint, use blue tape on the glass and get a nice line, hopefully let paint bleed under and the backside of the wood get colored, and that is proving to be moderately successful.

Another thought - the wood thickness is about 1/2" I could get some tiny wood trim, paint it, then glue it to the inside to cover up this angle a bit...like a tiny square strip of wood or something. Introducing glue to this sounds like it might make things worse so I am hesitant.

thoughts appreciated.

No good solution. Mirrors are glass with the reflective surface on the back. Anything you put on the front of a mirror will reflect off the back surface when you look at it at an angle due to the thickness of the glass. You can extend the wood farther over the mirror with something that is painted the same on the front and back so that you're just reflecting the same color. But if you look from a "flat" enough angle, you still might be able to see the back of the wood. Ideal solution would be to remove the wood, paint both sides, and then put it back, but it sounds like that's not an option.

Edit, show us a photo of the back of the door. Lot of the time, there's an easy way to remove the mirror, paint the wood, then put the mirror back in.

its a custom built mirror door, solid wood, mirror on both sides. it's heavy. not about to try and disassemble.
I guess in retrospect, take a qtip and paint the inside edge on the glass. I'll live with it as is but there are spots I can see that will drive me nuts forever