I have been the de-facto lighting designer on a few occasions.
Once because the LD that was hired was completely incompetent and the director and I knew that if we were going to make it through lighting sessions with an ounce of sanity I would be setting levels . . .so although I didn't hang and focus, I made a list of channels and started calling out levels, first as more of a suggestion and then just took it over. We lit the whole show in two sessions. It would have taken days otherwise. the LD didn't seem to mind as he couldn't possibly have cared less in the first place.
The other major one that comes to mind is an opera where a lighting designer simply was not hired, and I felt the production and the piece deserved more than "lights up/lights down". Being as that production had nothing to lose by letting me dabble, they agreed and I went to town. It was a credit for me and gave me the opportunity to learn, although it was trial by fire. When I wasn't in rehearsal I was hanging, focusing, and I did my own lighting sessions and lit over the top of rehearsals. Having almost no set at all, I'd like to think my design created acting areas and defined spaces a little bit more as well as popping color out when appropriate. Better yet, I had no notes from the LD during tech except my own. That production also toured, so I did the advance work on the next venue on what should have been my days off. I hold no grudges . . .I learned a ton during that process.
Lastly, I did a festival of short plays in DC many years ago. Again, just dabbling, but after you've heard LDs calling out levels for years, it isn't hard to figure out the basics, figure out what works and what doesn't, and keep the show moving forward.
I am honest, though, and never promise more than I can deliver.