Thrill-seekers told how a piece of metal broke away from the ride before landing near to them.
A gap appeared in the track and the ride was closed down.
In the queue at the time was 20-year-old Fiona Gilbert from Compton in Wolverhampton.
She and friends, Amy Cashmore and Farah Rahim, who are both 20 and also from Wolverhampton, decided to get in the queue for the roller coaster which opened earlier this year.
The University of Manchester student said: "It was about 10.15am yesterday and we thought we would start queuing for the smiler. As you queue you go through metal cages and the rollercoaster goes over your head.
"We were stood there when all of a sudden we heard a massive clanging sound. We turned around and there was a metal bar not even a metre away on the floor behind us.
"It was just under 1ft long and about two inches thick.
"The man in the queue behind us picked it up and took it to a member of staff and the ride was shut down."
She added: "I am never going on it. I just could not believe it and the whole thing really put us off the rest of the day. The worst part of it was that it could have hit someone."
The rollercoaster has drops of 60 metres and hits speeds of up to 53mph, it has 14 loops, which is more than any other rollercoaster in the world, and bosses behind its creation have said it 'messes' with the minds of riders.
It is the latest drama for the new ride since its opening.
In May, 16 people had to be rescued from the ride after it appeared to break down during a press launch and the planned opening was then delayed due to technical issues.
Alton Towers spokeswoman Elizabeth West said: "Yesterday morning, in line with our standard procedures, Alton Towers Resort closed The Smiler to investigate a small piece of debris that had allegedly fallen from the track.
"At no time was any visitor at any risk, and 48 guests on the ride at the time were safely disembarked. As the health and safety of our guests is our primary concern, the ride will remain closed whilst the resort's technical team investigate."