Bear with me; this answer is a bit complex.
I would not recommend that a client perform a contraindicated exercise.
Having said that…the word contraindicated crates a problem.
What exercises should be contraindicated?
A person with an ACL tear would probably benefit more from closed chain exercises (squats) than open chain exercises (leg extensions). In this case, leg extensions are contraindicated.
However, leg extensions would not be contraindicated for people who don’t have ACL tears and need to strengthen their quadriceps.
What Karin and Esta said is true… one exercise can be appropriate for one person but inappropriate for another.
Sorry for the long explanation.
Hope this helps you out!