The dumbest workout I ever did was 18 years ago. I ran a 5000 time trial in 14:50 and did three 300 meter cutdowns where the last one was all out and I could feel something bad happened doing that sprinting after a hard long fast effort. I was half limping on my cooldown.
The dumber aspect of the workout was I went back to the track about 6 hours later and did a second workout, a 4 mile tempo run at 5:20 pace.
The next morning I couldn't even stand up to get out of bed. I had to slide off on my back to the floor and get on my knees and crawl before I get stand. The lower back pain was excruciating. I had severe sciatic nerve damage.
It never completely went away. 18 years later at age 46 it still bothers me even though I only run about 3 times a week. Especially in the morning after I drive to work and have to brace myself to get out of my car.
All because of that stupid second workout that one day.