Supposedly they were not hardened properly. I think you can identify them by dragging a file across them. If they score, they have not been hardened properly.

This all went down in the late 90s, and the gold roller rockers were the biggest culprits.

I bought my rockers new off ebay just a few years ago. No indication that they were old stock. I may have just over-torqued an adjuster. It was broken mid-way up the threads. I got lucky that the broken adjuster didn't back out all the way and really create a problem. Just to be safe, I replaced them all.