Just realized I used a smaller stopper on my 23 litre carboy as it wasn't very snug (was sitting kind of loosely in the hole). Shoved it in and left for a week's vacation. Am I going to come home to spoiled cider?
most likely you will be fine. It is possible that wild yeast or bacteria got in and will give you an off flavor. Some oxidation could occur but it sounds like your getting a pretty decent seal. You'll know if your batch is infected either when you bottle (your bottles might explode or you might see infection in them) or when you try the final product (it will taste off). I'd be willing to bet your all good though.
i would think that as long as the cider is fermenting, there will be a slightly positive pressure of CO2 inside the carboy that would keep the air (or most of it) out. in my inexperienced opinion, you're fine.