Orange line metro riders had a scare last week when a car door remained open between the Vendome and Lionel-Groulx stations.
The STM is investigating the incident.
And while it's not the kind of event the transit agency likes to see happen, spokeswoman Amélie Régis says it is not entirely unusual.
"It's a pretty rare problem, but with the metro running 78 million kilometres a year, it's normal that there will be some mechanical problems once in a while," she says. "But this kind of problem only happens four or five times a year."
The incident occured on Christmas Eve morning, and several children were on the car affected.
Régis says the investigation will take a little longer than usual due to the holidays.