I figured out, at least for myself, that sometimes when the object you are controlling is suddenly moving on its own even after releasing any buttons - its because the event sheet you are working on has some ugly junk running and making the object ignore your input or lack of.
First things first, Ashley and others have mentioned this before - update your browser and gpu drivers.
After this is set, go into your event sheet and start trouble shooting by disabling events in your event sheet.
Test until you have found the culprit event that is making the object go bananas. Redo the event and make sure it structurally plays nice (or beat it into submission) with the C2 engine.
I have only had the issue whenever I test and I have an event with conditions structured in a weird way. I wish I could give an example of "weird" but it was pretty much the way things were organized and how things were being called and executed.
I have no idea if this will solve the entire issue for others, you may need to update gpu drivers or upgrade to a better gpu, redo events, update browser and make sure your problem isn't between the chair and the keyboard, I just know it has helped me and so far so good - I have had no crashes or any weird object movements.