Does someone know what this could be?

I had an MGB with a Lucas Electrical system. I got rear ended taking out the tail light and bulbs on the passenger side. I fixed it myself and put the signal light bulb in the slot for the brake lights and vice versa. Then I noticed that if my headlights were on and my foot was on the brake with the key in my hand the engine would not shut off until I either turned out the lights or took my foot off the brake. I fixed the problem by putting the bulbs in the correct socket. Have you done any electrical changes recently?