Rules
The FASTag e-notice and the 72-hour window
A failed crossing is not the end of the matter and it is not immediately a penalty either. There is a window — and what you do inside it decides whether this ever follows you.
What triggers it
Most commonly a short balance at the moment of the crossing. It can also follow a blocked or blacklisted tag, or a tag the gantry could not read at all, in which case the number plate capture is what identifies the vehicle.
The window
Where a payment fails at the plaza you are generally given around 72 hours to settle it. Settle inside the window and the record stays clean. Leave it and the amount is recorded as an unpaid due against the vehicle.
This matters most when a vehicle changes hands. Dues attached to a registration surface for whoever holds it, which is why closing a tag properly when selling a car is worth the small effort.
Why it matters more with barrier-free tolling
At a conventional plaza a failure is obvious — the barrier stays down. At an MLFF gantry there is nothing to stop you, so the first you may know of a failed crossing is the notice itself. As barrier-free tolling spreads, watching the messages becomes the only way to catch a problem early.