Oh boy I got a few. I could tell you stories about very stupid xss vectors like tracking IDs that get properly sanitized when they come through the url but as soon as you go to the next page and the backend returns them they are trusted and put into the Dom unsanitized or an error page for a wrong token / transaction id combo that accidentally set the same auth cookie as the valid combination but I guess the title "dumbest" would go to another one, if only for the management response to it.

Without being to precise let's just say our website contained a service to send a formally correct email or fax to your provider to cancel your mobile contract, nice thing really. You put in all your personal information and then you could hit a button to send your cancelation and get redirected to a page that also allows you to download a pdf with the sent cancelation (including all your personal data). That page was secured by a cancelation id and a (totally save) 16 characters long security token.

Now, a few months ago I tested a small change on the cancelation service and noticed a rather interesting detail : The same email always results in the same (totally save) security token...
So I tried again and sure, the token seemed to be generated from the email, well so much about "totally save". Of course this was a minor problem since our cancelation ids were strong uuids that would be incredibly hard to brute force, right? Well of course they weren't, they counted up. So at that point you could take an email, send a cancelation, get the token and just count down from your id until you hit a 200 and download the pdf with all that juicy user data, nice.

Well, of course now I raised a critical ticket and the issue was fixed as soon as possible, right?
Of course not. Well I raised the ticket, I made it critical and personally went to the ceo to make sure its prioritized. The next day I get an email from jira that the issue now was minor because "its in the code since 2017 and wasn't exploited".

Well, long story short, I argued a lot and in the end it came to the point where I, as QA, wrote a fix to create a proper token because management just "didn't see the need" to secure such a "hard to find problem". Well, before that I sent them a zip file containing 84 pdfs I scrapped in a night and the message that they can be happy I signed an NDA.