A pool of hydraulic fluid could have caused a crash which killed three young people in Lancashire, an inquest heard.
Jessica Foxley, her boyfriend Tom Petty and their friend Philp Wright died when their Ford Focus hit a wall in Colne in July last year.
A forage harvester, made by firm Graham Guy, had leaked on to the road earlier that day, Burnley Town Hall was told.
A contractor told the hearing he had sent out workers to put sand on the spill before the accident.
However, another witness, motorcyclist Robert Holt, said he had passed the pool of fluid and told the police because he believed it was an "accident waiting to happen".
He felt there should have been "warning signs on the road", the inquest heard.
The hearing is expected to continue for the next three days.
It has taken more than 16 months to gather all the evidence that it is hoped will shed some light on what happened leading up to the crash.
Ms Foxley, 21, Mr Petty, 25, and Mr Wright, 25, had been returning home from a wedding when their car span out of control and ploughed into a wall.
Since the fatal crash their families have organised various fundraising events in their memory.