At the time, it seems so important to do that last rep. One more rep, I can do it. Your prime movers are exhausted, so your postural muscles engage, doing the bulk of the work to move the weight, and you injure your neck or lower back. Now you have to avoid lifting for seven to ten days. Now, you can do that last rep if you maintain your form.
Form is everything.
If you have to sacrifice form in order to do those last few reps, you may get an unexpected, unwelcome result. Likewise in running. During training, when your form breaks down, that's a signal to slow down and recover your form. If your training is done with attention to good form and good mechanics, you will be more likely, when the time comes, to run a good race at a good pace. As in much of life, there's a fine line between training hard and overtraining.