It's a real thing appearing 12-72 hours after a heavy or new heavy exercise.
It's referred to as DOMS.
I've had it a few times, always when advancing either deadlifts or barbell squats. Soreness and stiffness that make you feel old and feeble for a few days. It is not about bad technique, because it's always been with supervision of a trainer or a PT for me.
It's not a bad thing, but short-term it is.
With exertion, the bad thing is rhabdomyolysis.