Even if Immigration denied your application for an extension (eg if you already had on extension) they would give you a stamp giving you 7 days to leave the counrty.
My experience of living here for well over 10 years is that Immigration will always take action if they find people working without an appropriate work permit. Period. Trouble: big fime, jail, expulsion from the kingdome. ban on reentry.
Depending how many times you come to Thailand, at some stage Immigration will pull you up because they believe that you are no longer a tourist and will want you to have a visa appropriate to your reason for staying long term in Thailand.