I set the front edge of the orb on a thin booklet which raised it maybe 2.5? and the behavior stopped. It sat there and happily charged all night. I'm content to find a little black cup or something similar to set the orb into so I can adjust the tilt, but that seems silly to me. I noticed this morning there is a wedge shaped round foot on the bottom which keeps it from sliding across the table top. The thin edge is in front. I suspect if that was turned around the orb would work flawlessly. I wonder if mine is on backward?
I've gone through two orbs and the first week or so it had no slippage. For the 2nd orb, I covered the face with a used gift card (credit card size) when my phone wasn't on it to eliminate dust/lint from accumulating. Still found slippage happening, though the phone was only sliding a few millimeters by morning instead of to the point where the phone made contact with the table.
Glass back, no protector, free of oils and such, same behavior with and without official bumper on it. I think the low winter humidity is reducing the friction between the orb and the back glass enough to cause occasional slipping.
So I tried the mod that someone posted on here earlier: removing the rubber ring on the bottom of the orb, removing the two nubs that are used as a guide to attach to the bottom of the orb, and reattaching it 180 degrees from the original orientation. Using the pitch meter from the Smart Tools app on the phone, I see that I have reduced the pitch of the orb by about 5 degrees, and this seems to be enough to stop all my sliding issues.
I think the pitch of the orb was designed to be as high as possible without slipping in lab conditions, but the tolerance is too high and becomes an issue for some users with less-than-ideal environmental conditions. It seems most prevalent for users without a back screen protector. Users with back screen protectors or cases have a higher coefficient of friction with the orb surface and the phone and rarely report an issue with sliding.
This is the survey I ran about a month ago.
https://docs.google.com/forms/d/1JSyLy3jWC1_WBJCc7qcJsofsTtzYAqTpi2WNUgMejUc/viewform
Roughly half the respondents reported slipping, and the majority of those have the bare back glass against the orb.