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I've found and fixed any kind of "bad bug" I can think of over my career from allowing negative financial transfers to weird platform specific behaviour, here are a few of the more interesting ones that come to mind...

#1 - Most expensive lesson learned

Almost 10 years ago (while learning to code) I wrote a loyalty card system that ended up going national. Fast forward 2 years and by some miracle the system still worked and had services running on 500+ POS servers in large retail stores uploading thousands of transactions each second - due to this increased traffic to stay ahead of any trouble we decided to add a loadbalancer to our backend.

This was simply a matter of re-assigning the IP and would cause 10-15 minutes of downtime (for the first time ever), we made the switch and everything seemed perfect. Too perfect...

After 10 minutes every phone in the office started going beserk - calls where coming in about store servers irreparably crashing all over the country taking all the tills offline and forcing them to close doors midday. It was bad and we couldn't conceive how it could possibly be us or our software to blame.

Turns out we made the local service write any web service errors to a log file upon failure for debugging purposes before retrying - a perfectly sensible thing to do if I hadn't forgotten to check the size of or clear the log file. In about 15 minutes of downtime each stores error log proceeded to grow and consume every available byte of HD space before crashing windows.

#2 - Hardest to find

This was a true "Nessie" bug.. We had a single codebase powering a few hundred sites. Every now and then at some point the web server would spontaneously die and vommit a bunch of sql statements and sensitive data back to the user causing huge concern but I could never remotely replicate the behaviour - until 4 years later it happened to one of our support staff and I could pull out their network & session info.

Turns out years back when the server was first setup each domain was added as an individual "Site" on IIS but shared the same root directory and hence the same session path. It would have remained unnoticed if we had not grown but as our traffic increased ever so often 2 users of different sites would end up sharing a session id causing the server to promptly implode on itself.

#3 - Most elegant fix

Same bastard IIS server as #2. Codebase was the most unsecure unstable travesty I've ever worked with - sql injection vuns in EVERY URL, sql statements stored in COOKIES... this thing was irreparably fucked up but had to stay online until it could be replaced. Basically every other day it got hit by bots ended up sending bluepill spam or mining shitcoin and I would simply delete the instance and recreate it in a semi un-compromised state which was an acceptable solution for the business for uptime... until we we're DDOS'ed for 5 days straight.

My hands were tied and there was no way to mitigate it except for stopping individual sites as they came under attack and starting them after it subsided... (for some reason they seemed to be targeting by domain instead of ip). After 3 days of doing this manually I was given the go ahead to use any resources necessary to make it stop and especially since it was IIS6 I had no fucking clue where to start.

So I stuck to what I knew and deployed a $5 vm running an Nginx reverse proxy with heavy caching and rate limiting linked to a custom fail2ban plugin in in front of the insecure server. The attacks died instantly, the server sped up 10x and was never compromised by bots again (presumably since they got back a linux user agent). To this day I marvel at this miracle $5 fix.