I remember my physics teacher telling me there was an error factor in e=MC2. As I recall it was up to 5% that they couldn’t quite find.
I suspect that you are not remembering entirely correctly. In any reaction, there is an "error factor" in that some of the mass goes missing. No one can find that missing mass.
Then Einstein came along, and in 1906 said that the missing mass no longer existed, it was converted into a burst of pure energy, by E=mc2.
By the way, very little mass goes missing in most reactions, but it is significant in nuclear reactions. More missing mass means more energy is produced.