Well... I can think of several bugs that I found on a previous project, but one of the worst (if not the worst, because the damage scope) it's one bug that only appears for a couple of days at the end of every month.

What happens is the following: this bug occurs in a submodule designed (heh) to control the monthly production according the client requirements (client says "I want 1000 thoot picks", that submodule calculates the daily production requirements in order to full fill the order).

Ideally, that programming need to be done once a week (for the current month), because the quantities are updated by client on the same schedule, and one of the edge cases is that when the current date is >= 16th of the month, the user can start programming the production of the following month.

So, according to this specific case, there's an unidentified, elusive, and nasty bug that only shows up on the two last days of every month, when it doesn't allow to modify/create anything for the following month. I mean, normally, whenever you try to edit/create new data, the application shows either an estimated of the quantities to produce, or the previous saved data. But on those specific days it doesn't show any information at all, disregarding of there's something saved or not.

The worst thing is that such process involves both a very overcomplicated stored procedure, and an overcomplicated functionality on the client side (did I mentioned that it dynamically generates a pseudo-spreadsheet with the procedure dataset? Cell by cell), that absolutely no one really fully understands, and the dude that made those artifacts is no longer available (and by now, I'm not so sure that he even remember what he done there).

One of the worst thing is that at this point, it's easier to handle with that error rather to redesign all of that (not because technical limitations, but for bureaucratic and management issues).

The another worst thing (the most important none) is that this specific bug can create a HUGE mess as it prevents the programming of the production to be done the next day (you know, people tends to procrastinate and start doing things at the very end of the day/week/month)... And considering that the company could lose a huge amount of money by every minute without production, you can guess the damage scope of this single bug.

Anyway, this bug has existed since, I don't know, 2015 (Q4?) and we have tried so many things trying to solve it, but that spaghettis refuse to be understood (specially the stored procedure, as it has dynamically generated queries). During my tenure (that ended last year) I spent a good amount of time (considering what I mentioned on the last rant, about the toxic environment) trying to solve that, just giving up after the first couple of weeks.

Anyway... I'm guessing that this particular bug will survive another 4-ish years, or even outlive the current full development team... But, who knows ¯\_(ツ)_/¯ ?