I want to make the moist lemon cake after work today (by makupwmn). It calls for half a can of sprite or mountain dew. Neither of which I have. Do you think I could just substitute some lemonade for that amount of liquid?

Yes you can. You can even use water if you were in a complete pinch. The carbonation does help add a nice texture to the cake but it's far from mandatory.

I make substitutions all the time with great results. The only thing to watch for is if you use an acidic liquid instead of something neutral. But then you only need to add a pinch of baking soda.