How to contest a transit fine

Assuming that this is for public transport in Brussels, you can contact the "Service de médiation de la STIB".

http://www.ombudsman.be/fr/ombudsman/card/service-de-mediation-de-la-stib

Just write to STIB and present your case. Identify the scanner as precisely as you can, indicate the hour you tried to use it, and remind them THEY need to ensure maintenance of their machinery.

You are most likely to get a letter reminding you that you a) may only use public transport when in possession of a ticket, and b) you needed to talk to the driver of the vehicle and purchase a ticke there. Which is of course reversing the responsibility of their badly maintained system onto the customer.

Nonetheless, if you are a first time offender, they'll reduce your fine to the price of a ticket.

Next time this happens to me, I will make a counter proposal to them: let's start a pilot project - whenever I detect a scanner that does not work, a selling machine that is out of function, a bus that carries no indication of line and direction, etc., I will issue a fine ticket of 100 Euro to them. 1000 Euro in case I find the situation unchanged after 7 days. That should serve as an invitation to keep their equipment working - and will probably make me very rich in a very short time. ;-)