Was some time ago - but, back in my PPL days, I was given a clearance to line up (at a large UK airfield) however the stop bar didn't go off.
I pointed out to ATC that the bar was lit - they said "It's OK, your are cleared to enter the runway". The bar still didn't go off.
Not knowing (and apparantly still not, to this day) if this was true I held short until the lights went out.
I'm curious to see where this thread goes.