But I think most people have a danger zone. This is the point where screw-ups start to get dangerous, and you don't want to be doing sloppy deadlifts or squats with a wobbly core. For some people, this might be two plates. For others, it might be the first time they have to reset their deadlift. Or it may be the point where you start getting minor injuries. (It may also depend on genetics: I have reasonably robust shoulders, but an unusually short sciatic nerve and easily-annoyed pyriformis muscles.)

If you're self-taught using videos or books, then when you reach that point, I would recommend paying for a serious powerlifting coach. A lot of self-trained lifters have way too little core tension, or are making other dangerous mistakes. Personally, I worked with a Starting Strength certified coach who was excellent, and who fixed all sorts of mistakes I was making—but it still wasn't enough.

Depending on how completely I recover from this injury, I might just freeze my deadlifts and squats at 225x5 permanently, and focus on a safe subset of bodybuilding lifts. But I do recommend talking to older lifters, and asking them about how they avoid injuries. I've known some admirably buff 55- and 65-year-olds with good injury histories—but they seem to focus on a combination of moderate strength and aesthestics, not on raw powerlifting totals.

Overall, weightlifting is statistically one of the safer sports. But injuries are just a miserable experience, and I encourage people to think about these issues before they're forced to learn about spinal anatomy, MRIs and anti-inflamatory drugs (or about rotor cuffs).