Things that I have felt just before going down;
A completely blank feeling from the front. - Tucked front due to excessive corner speed where the track changed camber.
The bars wanting to turn in all on their own - Tucked front due to corner speed greater than available grip.
Questionable traction from the rear - Lost rear due to overheated, worn out tire. :
I do not blame the tire, it was my fault because I knew better... but rode fast anyway.
Front chatter (light to moderate vibration in both bars) - Tucked the front due to being tight on the bars over rippled tarmack.
Hard rear chatter - Lost the rear when tight on the bars when pace was greater than the tire could flex, tire was most likely over inflated as well.
Warning signs that I was nearing the edge;
Chatter, both front and rear
Mushy feeling from front and/or rear
Rigid mid corner feel from the front forks (most cold track days, bad springs and/or valving)
Various slides/pushes from front and rear
Overall instability - that feeling you get in your gut and lower torso or that "about to fall" feeling. (wet track days and very cold track days)
Rear spin ups - wet or cold and (super hot track days on a crappy tire). Note: throttle power spins ups are not necessarily a traction limit indicator but can send you down