The worst code I remember seeing right now is the gnugo code (the Go game). A total lack of control in the monitoring, too many functions accumulated that do the same and duplicate variables because nobody knew what the previous ones were doing.

@Edgar Reynaldo :
Curious, I have never needed to rewrite. I am so perfect that I have already forgotten how to be wrong.

Wow.

Some weeks ago I had to refactor manual XML traversing code that would randomly crash. Turned out it exclusively matched attributes to identify elements, ignoring tags. Besides, there was a preference for matching attribute values without matching their names. Also, things like getChild(3) were used extensively. It went down a tree like this. On the way, it used some obscure helper functions to match elements that did entirely different things if empty string was passed as the fourth argument and so on. Of course, all constants were "inlined" with no regards to eliminating duplication. And yes, sometimes, the same constant was spelled a bit differently.

Not professional, but you should see some of my early code.

I was very fond of single letter variables. Plus, Pascal is case independent so everything is capitalized.

So you use pragma once or all your code is in 1 file ?

Or ForwardDeclareParty ?

Definitely not the worst that's out there, but my codebase at work is ~20k lines in embedded C, everything's in one file, ~300 functions that all return/take void, and the biggest functions are 1k lines long, ~200 global vars, nested switches, and if/elses nested 7 levels deep.

EDIT:
I almost never rant, so this is a rare occasion.

There is so much dupe code in our base, it's not funny. Often, people enable or give excuse for low-level languages to yield poor spaghetti code, but there really is no excuse. Unless you are developing for a device that either doesn't have a processor with a call stack, or at the very least, a jump instruction, there is absolutely no reason for you to duplicate code...ever, and no, loop unrolling to get more cycles is not a reason in this day and age.

I've consumed a lot of alcohol over the years to forget, but I've encountered some terrible code in my travels.

When I first started working in the industry I was working on a VBScript-based ASP Web application that had zero code reuse, duplicating everything as needed (even within the same page/program). Additionally, the original authors didn't understand the separation between server-side and client-side so the code was mixed all throughout the document pages in nonsensical ways. And worst of all, every page was vulnerable to SQL injection in every single query that would just concatenate user inputs into the SQL command. It wasn't uncommon to find code that didn't even do what it was intended to do.

Another notable example that comes to mind is a Web application that was written in a proprietary IE-only scripting language (I can't remember now if it was VBScript or a superset of JScript or something else entirely). It was developed originally in Spanish too which didn't help. And the code was complete spaghetti. I was asked if I could make changes to it. I wasn't even able to begin to understand how it worked. The person or team that developed it was long gone, perhaps on the other side of the world. Who, what, when, where, why, how? Nobody knew. I didn't know anything about it other than where the code existed in the company network. It was a large international corporation and the code had been shared from a different location. I forget what, but I did manage to make a minor change that I guessed might accomplish a backwards, compromising, partial goal. It was a really big hack, and I couldn't even prove that it worked, but the guy requesting it was excited about it nevertheless...

I know you're shitting me, but I'll humor you. The password was stored in a publically accessible database.

Name it .dat and you've defeated 3/4 of the world.

Literally everything I've ever touched professionally has horrified me in some way.

4 SQL servers with 24 cores (when they FIRST came out, over $30 grand a computer), all running their SQL database off a USB 2.0 5400 RPM HDD. "so they can remove it easily for fire safety."

Gigabit FIBER around the entire compound. Then tied to a T3 line (which is like 300 KB/s) and the sales techs complain "Microsoft Dynamics is really slow when used remotely." Their goddamn CELLPHONES have faster internet.

I felt like Scotty every day. "I'm a programmer, not a MAGICIAN"

More IT, there's a college that lists their entire site's root passwords in a plaintext file available ONLINE so Norton Ghost can retrieve the ISOs when they install new computers. (plz die.)

I haven't even started on the software I had to maintain.