Residence permit

Does anyone know the ramifications of not renewing or replacing your residence permit? I have lost my expired res. permit and have attempted to replace it. Went to 1000 commune, was told I needed some paper from police verifying that I had lost my card. Went to police got the paper, with two photos stapled to it. Took said paper back to commune only to be told I need some yellow paper signed by the police otherwise it would take many months to get a replacement res. permit. Went back to police, got yellow paper signed--after the copper had to come to my house to make sure I really lived there. I have been in Belgium for 12 years--they clearly have me in the computer because the original attestation from the police (that I had to take to the commune as proof my card was lost) was printed out from their computer--with all my details. Now I'm fed up and can't be bothered to go back to the commune. In 12 years I've never been asked to see my res permit. What would happen if by chance I was asked and did not have it--but did have other identification.