Rode my motorcycle for probably 6 months with an expired registration sticker on it. Never noticed and never pulled over, just happened to see the new one laying around one day and realized that I never put it on.
Oh, and I drove my Cavalier for a week with a "borrowed" registration sticker from one of our other vehicles while I waited for the cat monitor to set itself (found out that it sets immediately after clearing codes, but takes 5 warm up cycles after replacing the battery :/) so I could pass emissions and register it legally after it had sat in my yard for 3 years. Following the "correct" process would have cost me money and turned into a bureaucratic pain-in-the-[censored] (go to DMV, wait in line, get one-day permit, take to smog place, pay for smog check, obviously fail for incomplete monitors, go back to DMV, pay $$$ for 30-day permit to get monitors complete, pay for another smog check, pass, go to DMV and wait in line a third time, finally get registration). I only drove it around the rural area near home so I figured it wouldn't be an issue.