It depends where you are. If you're in the UK and on a road, there's a whole load of offenses. You're driving otherwise than in accordance with a license, no insurance (Presuming you don't have any), no tax, no helmet..... the list is quite extensive, especially if there's any defects on the bike. Then if you're driving else where than on a road (Over fields etc) you need the permission of the land owner or it's another offence of driving elsewhere than on a road.
If it falls into any of the categories above (No license, no insurance, elsewhere than on a road or is considered to be careless driving under the Road traffic act 1988 sec 3) the bike can be seized by the police, you can be issued with a fixed penalty for whatever offence has been committed (Or reported to the court) and you can be issued with an antisocial notice in relation to the use of the vehicle. The notice is attached on the police national computer to you and your vehicle. If it gets 2 warnings, again t can be seized. Similarly if you;re driving another vehicle and have already had a warning on your bike and get stopped again and you're given another warning, because you have had a total of two, the vehicle that you are now using can be seized!
It's fairly complicated but basically you shouldn't use a vehicle at all, unless you've got the correct documents for it.