Luas validators have been fixed after it emerged some customers were wrongly being charged peak fares.
A number of the machines where passengers tag-on were found to be running two and a half minutes fast.
It means validating your journey at 6.58am would see you incur the fare for travelling during rush hour.
Tech journalist Jess Kelly, who broke the story, says Transdev is blaming 'clock drift'.
She said: "The time on these validators are set by an automatic fare collection system, and that's run by a central support system, so this is an automated process.
"And they said that it is possible that PC clock drift can occur.
"So for those who don't know what clock drift is, sometimes if your computer does a software update, it can be knocked out by a few seconds, or indeed by a few hours."