Yep, happened to me a couple of times now over the course of 2 years.

I ran the engine upright in a big stick airframe first, and used to run the fuel out and put some oil down the throttle barrel at the end of the day. Never had a problem with this until I mounted it inverted in the Aries. On the very odd occasion it just wouldn't "clear" itself for the first start of the day and needed the plug to be pulled. Seemed to coincide with being a bit heavy handed with the oil the previous weekend. I stopped running out the fuel and putting oil in the motor once I'd put 2 and 2 together.

Reassembled the engine with too much oil, and had the needle and reg a touch rich. Ran OK for about 2 minutes until I advanced the throttle then loaded up and died. Again it would start, run but wouldn't clear itself, it got worse until it wouldn't even pop on the starter. I pulled the plug, blew it out, fired first pop then I picked up the plane and held it upside down running while it cleared out the excess oil.

Just a side note, I tend to spend a fair portion of my flying upright, cruising around on part throttle, a 50/50 mix of sport flying and pattern practice. When running 20% oil I found a couple of outside loops with full throttle at the top sorted out any little misfires before they got bad. Running 10% oil has eliminated the need for me to do this. I'd tend to think that if I just took off, flew the F3A schedule and landed again (ie. used the engine for it's intended purpose) then oil fouling a plug wouldn't be an issue.

Another side note, even running 10% oil I changed the plug on the weekend before heading out and after a week on it's wheels the old plug was full of oil. I'm sure it would have started and ran just fine like always, but yep, definitely full of oil...