At a previous job, when I transitioned off a few projects that I had solely worked on for 7 years, I warned the dev taking over from me: I don't usually comment

At a previous job, when I transitioned off a few projects that I had solely worked on for 7 years, I warned the dev taking over from me: I don't usually comment my code, but when I do, you'd better pay attention. I think in one of them, only a single source file (C++) had comments, and there were more comments than code. I was explaining an intricate locking pattern around a data cache. Like you suggested, I added the comments after I'd done the work because it was a work in progress. I added the comments because it was a very fragile piece of code and sensitive to changes (e.g. really easy to cause a deadlock or race condition if changed).