Your experience may vary. Around here, they can't retroactively cancel coverage, just going forward from the date of an alleged violation. Likewise, to deny coverage they would have to show either it fell under an exception/exclusion that was in place at the time of loss (both within the binder/policy documents, AND that the excluded condition contributed to the casualty event.)
Layman's terms - if something was excluded from coverage (unregistered/uninsured auto) when the fire happened, it's out. If something was listed as excluded (wood burning heat in garage) and it caused the loss, no coverage existed. If something was prohibited (gas tank in garage regardless of gas mix) but it didn't cause or contribute to the damage, then it's not grounds to void coverage for the loss, but, if the policy allows it, could be grounds to cancel going forward.
Again, you'd need to talk to someone local who knows more about your jurisdiction.