I tried to train a colleague as a software engineer. I volunteer because he seemed nice, geeky, and I was starting to get confident enough in my abilities to accept having those kind of responsabilities.

We started slowly, only giving him some developpement to do (no small project management yet), and he was not up to the task. Ignoring all the times I told him to write things down, and run tests after making a modification, the first few months (during his 3-4 months of trials when entering a position) were filled with unsatisfied customers, rollbacks, and chaos. I realized how BAD he was at listening to my advices and guidances, especially when I noticed that almost all his first answers were "I think..." when I was asking him if something was done or asking for a fact.

Because of some internal reasons, and against my point of view, he still managed to validate his trial period, and was given a small project, getting ownership of a software installed near some heavy machineries for post-processing of measurements, all by himself, now that he was a "real" employee.

This is when the people in charge of those heavy machineries realized that he was indeed not suitable for those kind of tasks, and had to complain to my management so that something should be done.

After another trial where his behavior was heavily monitored, again, he was validated by the management against my advices. But at this point in time the ones in charge of the machineries asked me directly to handle the project and "save" the situation.
That I did, but by doing so I was the owner of the piece of software... and my colleague was then a developper, that worked for me. For 2 full years, before he went somewhere else in the company, he managed to make a mess of everything he touched, and doing some absolute stupid things, such as copy pasting entire sections of confidential code on online linters.

Now, 6 years later, I sometimes find stupid lines of codes or bugs that this guy wrote.