Mine was a security issue.

When I first got into backend programming, one of the first things I did was build my own authentication system (sign up, sign in, etc.)

And...I stored those passwords in plaintext. Yep.

But I learned from it later on, and now all my apps use hashes with salt to store the passwords.

It turns out Facebook had the same problem when it was first created...

...and they had one password that could be used by Facebook employees to sign in to any account. Yikes.

Quick tip: If you ever go to a website's "forgot password" page, and they email you your password, that website has terrible security.

Boy... I don't know where to begin. I've broken just about every rule in the book. My main problem i think is I find I use if statements without the else a lot. Like I will do this:

foo = 300

if foo == 200:
    print("foo is equal to 200")
if foo is != 200:
    print("foo is not 200")

I could of easily used an else there, it also looks extremely messy.

I name my variables ridiculous names.

I just give them names off the top of my head.... I'm trying to break that habit but I will literally use "foo" as a variable name if I can't think of anything.

If I listed everything else, I would go on all day. ;D

Good question and several useful answers so far! Here are some of the bad practices I have encountered:

• Mixing early return with if-else

if (x) {
  return 1;
}
else {
  // Do something else
}

• Inconsistent naming e.g. variable name looks like a boolean, but you store something else

var isPublication = "publicationId";

• Making code harder to read and complicating it unnecessarily

return (x === 1 ? true : false);

• Lover of callbacks e.g. nesting too many callbacks when you can easily avoid them using something like async/await

dosomething(10, 12, (data) => {
  doSomethingElse(data, (something) => {
    doAnotherThing(something, () => {
      // More callbacks
    });
  });
});

• Making typos and copying the typos everywhere because you couldn't notice. Sometimes you misspell a variable name and copy-paste it to different places -- it works -- However, when other collaborators glance over the variable name and try to use it in some section, the code simply doesn't work. It wastes a lot of time because we are not able to figure out that the original variable name is incorrect.

• Failing to return early by mistake. Some developers accidentally skip return keyword while returning early in case of error.

if (err) {
  res.status(400).end(); // This doesn't halt the execution of code
}

// rest of the code

There are many more, but these are the ones I've come across most frequently. I have always highlighted them in the code reviews so far. :D

I'm a React Native Developer so my answer will be in that context.

1. Global Variables: I used to define at the very start of my career.
2. Arrow functions inside render method: For the first 4 months, I think. Then I had to implement a FlatList and optimizing it taught me a lot about rerendering.
3. Inline CSS: It's a good practice to define a StyleSheet outside the render method because it keeps taking memory on every rerender.
4. I used to write code with proper indentation and format then I enabled format code each save setting in VS Code and it's so much productive now.
5. Using for loops: Can't say it's a bad practice but using Array Higher-Order Functions like map and reduce is so much efficient and code is more readable too.
6. Using recursive functions everywhere: When I first learned Recursive, I started using it everywhere which is bad. If the stack is going to be large then it will take a huge amount of memory and you might run into stack overflow. It's important to know when to iterate and when to use recursion. Yup, that's pretty much it.

A function that loops all of the array of objects, then inside of it has also a check if the current iteration of that loop is an array or object, if it's an array it will recursively run itself again, and returns nothing. Imagine you have hundred thousands of data / object that is being passed from that function and it returns nothing, no mutation at all.

I want to pinpoint one project, where I would say that I have never seen the worst coding practices anywhere else.

1. Using scoped styles in every component
2. Using a JS framework along with defining custom components (like: buttons and labels)
3. Not using ESLint or any other linting tool
4. Spacing and tabs mixed together
5. Not removing old/unused code

More of a deployment/version control thing but I've seen people version control the code and then just copy the files to test + staging servers LOL. I've done a couple of bad things in the past but nothing quite like that.

That's probably the worst I've come across.

Hmmm it was with PHP 4. This is just an example how it looks it has been quite a while

function myMethod() {
  $var1 = 12;

  DifferentObject::myOtherMethod(&$var2);
  $var3 = $var2->doSomething();
}

and the fun part was the reference was sometimes passed through 4 - 5 layers.

This was quite a confusing way to do things.

It's not that I am innocent of doing horrible things with code but this example actually kinda scared me ;D ....

I also experienced some bad coding standard ..

a. Very convenient way to declare global variable into program -- Memory leak issue . b. Use ForEach loop like a bread & Jam -- Performance issue c. Always ready to implement everything in UI layer rather then think twice and move to right place . d. Use proper design for implementation , It can avoid lot of bad practice .

Still Counting :)