Hey guys! Just wanted to share a nerve-wracking experience with you guys for input and so that hopefully you'll know what to do if it happens to you. 

TL;DR at the end for no spoilers

I was riding home on my DT3 250 and I thought I noticed the throttle sticking on the last couple blocks to my house. I got in the driveway and wanted to check and see if I was just imagining things, and so I revved the engine a bit and it felt a bit "drifty" (i.e. wouldn't slow down as quickly as it should). I revved it to about 8k and sure enough it got totally stuck. I assumed I had a bad throttle cable and I was glad that I found out when I was in my driveway and not on the road. 

This is where things get interesting.
I switch off the keys of the bike, but the bike doesn't stop running. Confused at this point, I unplugged the black wire from the stator to no avail. Finally I took the boot off the spark plug and realized that I had an engine runaway on my hands. I had heard of this happening to diesel engines when oil leaks from the crank case or valve train into the engine cylinder, but I had no idea that this was even POSSIBLE on a two-stroke. Everything that happened so far occurred in a span of about six seconds, and all the while the engine is screaming away at a steady 8k RPMs. My next move was I shut off the fuel and then eventually disconnected the tank (to be sure), but even this didn't stop it. I remembered that it could be running off of the engine crank case oil and so I assumed that shutting off the fuel wasn't helping. My last move was to try to plug the air intake, which is what I've heard is the solution for the diesel trucks. I proceeded to flip the seat and wrestle off the fuel tank to reveal the carb and intake, but the intake is a sealed unit that I couldn't just shove a rag in. I spent a solid ten seconds trying to wrestle the intake box out of the bike, but apparently the once-loose intake boot clamps I had affixed were suddenly gorilla gripping this thing onto the carb like their lives depended on it. I was at a loss for the moment, I thought I would try the choke for a second (I didn't jump for this sooner as the engine still runs fine with the choke on for me normally) and the RPMs just jumped from 8k to 9k, but then they suddenly fell. 

My first concern was that I had just shredded my engine. I pressed the kickstarter and it felt like it still had compression, so that was good. Turns out I had literally just run the engine out of gas, the runaway after I shut off the petticock was just residual fuel in the carb. I reconnected the fuel tank and ignition system and it seemed to start and run fine. I was very relieved and I went and had dinner. 

So anyway, now I know that that can happen! Anyone know what causes that and how I can prevent it from happening again? Should I check anything for specific damage? I hope I can spread this information to someone who was unprepared like I was, I can't imagine what would have resulted if I was on the road when it happened. Thanks all!

TL;DR Bike encountered runaway condition, tried a bunch of stuff, shutting off fuel worked. Bike works fine now I think, want more info/advice.

This happened to me on a DT3 and an RT3. Would not stop with the key switched off. Had to put them in gear and let the clutch out with the brakes on.
Two of the large head bolts loosened on the RT3. The 4 smaller head bolts were still tight maintaining compression. The cylinder assembly was loose at the base and caused an air leak. This was due to a thicker aftermarket base gasket installed by the previous owner.
I never did find the source of the air leak on the DT3.
Both suffered no damage but I did replace the base gasket on the RT3 with OEM.
These engines are tough.

And I had a stump grinder with a Deutz diesel that flipped over and went into a runaway condition burning the crankcase oil. Had to hold the exhaust flap closed with a rock while it screamed at high RPM spraying me with hot oil. Took over a minute to smother it. The engine was destroyed.

I'd have thought for that to happen there'd need to be throttle stuck open to get fuel. Fuel has to come from somewhere. But would need a very hot spot or glowing plug tip. Must admit i've never ever had that happen thank goodness.

For it to rev that high, air had to come from someplace. I've never seen or had it happen with a stuck slide, key shuts that off normally.

The not coming down to idle you described could also be an air leak.

At least with a bike you might have the chain on and be be able to stomp it into gear and stop it. I had this happen on a JETSKI 800 SXR and I was blowing the water out after riding on a hot day and I started it on the trailer and blipped the throttle and it went wide open. Pulled the tether , nothing Took the hood off and the only way to stop it was to cover all the intake vents for the carb. Meantime it ran wide open for 30 seconds to a minute. I believe it damaged a rod because next summer I was going down the lake and a rod broke and went thru the bottom of the engine. 2500 bucks later good as new.