Had the good fortune to fly jumpseat a number of times.
Most eventful was on takeoff from Buenos Aries. Taxying out to the hold, whilst deploying the flaps for take off, we received a red warning signifying a problem. Captain recycled the flaps, indication disapeared. Captain briefed the F/O to stop if the warning re-appeared below 80 Knots, but continue if it happened in excess of this.
Off we go and guess what happened @ 135 knots, not to far short of V1, yes a red warning. Captain calls "STOP". and by god did we stop.
The aircraft was a BA 747 400, very heavy, I was in the centre jump seat and I videoed the lot!!!
We had a brake overheat returning to stand and ended up stopping another night.
Cannot remember the skippers name, but he has a copy of the video for his own private re-run of his briefing.