I open my oven all the time, never had a problem.
I don't use a timer so I judge doneness by smell and touch and finally a skewer in the middle, sometimes I open it as much as 5-6 times.
Unless you kept it open for a couple minutes or slammed the door shut when you realized what you'd done, I'd guess that nothing's going to happen. Wouldn't make a habit of it, but I don't think it'll hurt. Most modern recipes are way more resilient and modern stoves return to full heat much faster than the wood stoves our grandmothers used.
I open and shut the door all the time during baking. I even take the half-done cakes in and out to move things around if I need to. It won't kill them.
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