This actually happened to me on a B734 a few months ago. We taxied onto the runway and immediately commenced the take off roll. Within about 5 seconds the engines spooled down and I glanced back from my seat (1C but single class!!) to see that the L/E devices were retracted (couldn’t see the T/E devises) after rolling down the runway for about 10 secs the power was applied again and I again looked back to see the L/E devices were now extended. However after a few seconds the power was then reduced followed by another few seconds rolling before the power was reapplied and we took off.
What can you do? Nothing. You have to rely upon the crews training and aircraft safety systems for your own safety, else go by train or boat.
That's not to say I was overly happy about the situation and this carrier is now on my no fly list.