I'm trying to think of some hypothetical situations, it gets difficult since usually karma is very complex.
Think of the events of your life, how might something that has happened to you in the present come from your actions of the past?
All I can give are actions with immediate consequences. Play with fire, you get burned. Put a fork in the toaster, electrocuted. Cut class, get suspended. Find someones wallet and return it, get a reward. Do good work, get a promotion. These are physical actions with consequences, and there are plenty of them occuring in your everyday lives.
I never claimed to have definitive proof, just a possible explaination of things that occur to us throughout our lives. If some physical and mental actions have immediate consequences, then other physical and mental actions must have delayed consequences, even if these consequeces are facilitated by other factors. Think of every single action you make, everyday. There are a lot them, most of them insignificant but even if the probability of that action having a consequence is small they may add up to something eventually.

Goin to smoke another bowl.