When this has happened to me I've changed the schedule without any problems. So if I started with a schedule of lights on from 8am-8pm and then one day i accidently left them on until 11pm then I wouldn't turn them back on until 11am the next day. From then on out my schedule would be 11am-11pm. I definitely wouldn't want to try this more than once with a plant during flowering, and I'm certainly not saying that there is no chance of this causing a problem...I'm just saying it's never caused a problem for me. And if your setup doesn't have the ability to block out natural daytime light then changing the schedule isn't an option.

I'm interested to hear what other people have done when their schedule has been thrown off by mistake.