If you overdraft your bank will impose you a "fine" and send you (at your cost) a registered mail of information including a command not to draft any other cheque before the situation is corrected. Drafting a cheque after such a command is a "délit" (Art. L163-2 Code monétaire et financier) with a possible sanction up to 5 years of prison and a € 375000 fine. To clear the situation the "offender" should either pay the amount of the cheque to the recipent in exchange of the cheque which will be returned to his/her bank or pay a provision to his bank (secured on a separate account) to cover the amount of the cheque. Another sanction for overdrafting can also be the interdiction to draft cheques for 5 years.
Legally, a cheque is not valid anymore 1 year and 11 days after the date of emission so if you forget to deposit a cheque for too long it will be lost...