Just while we're on it - the most important word in here is, individual. Granted, if you're a coach/trainer the majority of clients you have are going to be physically in good health with a minor niggle - but i've seen many a trainer in my time do potentially hazardous things in regards to training when it comes to an individual with a condition (type II diabetes mellitus, pre existing heart condition, bad arthritis etc).
If you have an older adult with bad, existing arthritis in the knees - you don't tell him to squat ass to grass when he tells you he's in quite a bit of pain. Back up, re-assess the situation. Yep, have seen it happen.