Over training can occur, of course, however many people do not train intensely enough or often enough to warrant undue fear of it.
You can hear all kinds of differing opinions on it, like anything else on the Internet, just keep in mind it is a possibility.
I am more inclined to think that people are not getting enough recovery time and quality sleep. The strongest people I know all stress the importance of those two things and they validate that by what they are actually able to do.
Yes, proper nutrition is a vital part of the equation, but I want to emphasize the other aspects here.
In this chaotic world we live in, stress is at an all time high. It definitely can detract from
both the quality and quantity of your recovery and sleep.
Social media addiction, watching TV (especially the news), playing video games late into the night all hinder your recovery & sleep. There are more things that could be added to this list, but you get the picture.
Serious athletes/lifters know how to dial in their recovery/sleep to maximize their gains.
If you want to take your training to the next level, take a good honest look at your recovery and sleep habits. Change whatever you need to in order to be the best you can be!