Question

What are some explanations for a yield going above 100%? Referring specifically to a yield produced when re-using products from a previous experiment (re-using recovered NaI to produce CuI).

Answer 1

You can have experimentally over 100% easily, I have it all the time in my lab. This happens mostly because it is not dry, if there are still extraneous solvents when working with microscale reactions, it can really mess up your yields. essentially if it's over 100% that means you have impurities.

to have less than 100% (which is probably the most common). Every time you transfer a component or reaction to another container, you lose some of your compound. A basic rule of thumb is to minimize transfers.
Another way to have less than 100% is due to equilibrium. If you are just running a straightup reaction, it'll be pretty much impossible to get 100%. This is because you will reach the equilibrium point of the reaction. You have to modify the equilibrium to get 100%.
Another way to get less than 100% is by stopping the reaction before it has completely reacted.

And of course the catch-all: Human error. Say you can't read a graduated cylinder right, or forgot to tare the scale before you used it.