The most crazy issue I've fixed was caused by a TCP behavior which I didn't know, called the "half-closed connection".
There was a third-party application installed on a production server which called a LDAP server for retrieving users information. During the day we had several users using the application and all worked fine. During the night, when the application was not accessed, something happened and the first call to the application in the morning was stuck for about 5 minutes before returning a response. I tried to reproduce the issue in a testing environment without success. Then I discovered that the application and the LDAP server were located on two different networks, with a firewall between them. And firewalls sometimes drop old connections. For this reason network applications usually implement a keep-alive mechanism. Well, the default LDAP Java libraries don't set the keep-alive on their connections. So, I found a library called "libdontdie", which force the keep-alive on the connections. I installed the library on the server, loaded it at the startup and the weird stuck behavior in the morning disappeared.

!rant && !question

If anyone has some info on this behaviour :P

I've got this website and it loads fine and stuff, but for some reason when I pop chrome into mobile mode on my desktop (you know the mobile emulator thingymabob) and reload the page the server returns a 500 error, like how does that even work?

(works on an actual phone btw, safari, edge, IE, Firefox, ....)

FYI I use lighttpd as a webserver

So here is a mini rant from an amateur/hobbyist developer (me).

Over the past week, I've taken on a project that is much larger than any other projects i've attempted to handle (steam trading bot). This meant that there would be logic flaws, weird bugs due to unexpected behavior from shitty web apis (and their poor documentation hmmmm).

Anyhow, fast forward a few days and the code is complete. It's mostly functional, apart from a few glitches and unexpected behavior here and there...or so i thought. Apparently if someone trades and item to me that isnt in my pricegrid, the bot freaks out and kills itself, relaunches, and repeats this cycle (pm2). And i only found out about this on my way to school

So in desperation to fix such a critical flaw in my code (if my bot breaks a lot and doesnt accept trades, i can get banned from backpack.tf), i bust out my only device which is my phone, and start editing away (JuiceSSH and turbo client is godsend ty). 30 minutes later, after toiling through code with no indentation or syntax highlights (mobile pls), ive fixed it. So i push to live and alls well.

Then I arrive at school, pull out my laptop and decided to check up on my code to see if anything needs fixing.

Oh look in one line i used '||' instead of '&&'.

ok lets fix it.

ok lets push to live again.

I launched WinSCP to move the files onto the server, and just as the loading bar finishes and the file is overwritten, i realized; FUCK the code i had on my laptop wasnt the latest version i just worked on on my phone.

So that's that. 30 minutes of typing code without indentation and syntax highlighting on a 5 inch screen and it's all gone.

TLDR:
Version control is a must.