Vehicle insurance.

It depends. Not having a valid certificate doesn't automatically make your insurance invalid.
However, if you have an accident due to something faulty that could have been dealt with if you had a CT done, then they may have a right to void the insurance.
For example, the brakes are tested in a CT. If you had an accident due to faulty brakes, and you didn't have a valid CT, then they may say you were at fault.

Take a look here under "Droit de recours de l'assureur" at the bottom of the page:
http://www.goca.be/fr/p/Sanctions

If your car fails the CT you will be given a certificate valid for long enough to get repairs carried out unless the failure is so drastic that the car has to be taken off the road so your insurance still covers you.
If you simply do not get the CT done then you are not covered except if you can show that you are on the way to a garage to have work done or to the nearest CT station to have a test carried out.
And all this applies whether or not you are involved in an accident; you can be stopped by the police for a roadside check, for example.

Remember that you are much more likely to be stopped by the police - or the customs/tax enforcers, who are also out and about on the road - if you don't have a valid CT certificate when you should. Your car registration number will be on the list of vehicles to be stopped and, with many police, etc, cars being fitted with automatic number plate recognition cameras these days, the chances that you will be spotted are quite high.

If you don't have one and your involved in an accident the other people insurance may well use it as a reason to not pay your claim I'd suggest driving a minim time without one whilst getting repairs is the safest option