This is every weightlifter’s nightmare. Never use free weights without someone to spot you.
I once was told by a coach that I should never lift weights alone, but if I did, I should not use collars (there are spring things now to hold the weight on the bar) so you can dump the weights off. However, I've seen guys power lifting and hurt themselves badly enough that I don't know if they could have crawled out and found help fast enough. This is terrible, and I know that poor planning could have landed me in the same place when I was younger. Supervised weightlifting is great for high school and college aged kids, but unsupervised weightlifting can lead to injuries, and lifting without a spotter can very likely kill you.
I don't think there is anything wrong with lifting free weights alone as long as you don't lift near your max repetition or max weight.
A mechanical failure of the weight rack is something a spotter won't be on guard for and will leave you seriously messed up regardless of how many people are present to watch you suffer.