Sometimes when Holmes is questioning someone you'll have the option to press
within a short (but reasonable) time limit and contradict the witness by bringing up a particular piece of evidence. If you pick the wrong one, you're notified of your error and are given another chance to choose correctly. It's often fairly obvious which one that is, but you can use the process of elimination if you need to.
There are well over 10 of these in the game so you shouldn't have much trouble. I unlocked this achievement while still on the second case.