Vertically-oriented O-rings
Lubricating O-rings is certainly something I expect you all do. This makes installation easier and also non-destructive—dry metal catching an O-ring can kick off a leak moments after it’s installed. Nothing here is news to you, I am sure. Lubricating with the sealing substance is always a safe bet.
However, when confronted with a O-ring that is oriented vertically when in service, grease is the bee’s knees. The high surface tension of the grease is almost always enough to hold the ring in its groove without falling to the floor, allowing you to line up parts, start fasteners, and generally avoid fighting with the sealing item.
Do your homework on the O-ring material, of course. It may be that you need a silicone grease or something with a more exotic base than the dinosaur goop, but as long as the materials are compatible, grease serves as both an adhesive and a lubricant in this role. I’ve had especially good luck adding a dab of silicone-based grease on long, finicky water pump seals.