Worst thing you've seen another dev do? So many things. Here is one...

Lead web developer had in the root of their web application config.txt (ex. http://OurPublicSite/config.txt) that contained passwords because they felt the web.config was not secure enough. Any/all applications off of the root could access the file to retrieve their credentials (sql server logins, network share passwords, etc)

When I pointed out the security flaw, the developer accused me of 'hacking' the site.

I get called into the vice-president's office which he was 'deeply concerned' about my ethical behavior and if we needed to make any personnel adjustments (grown-up speak for "Do I need to fire you over this?")

Me:"I didn't hack anything. You can navigate directly to the text file using any browser."
Dev: "Directory browsing is denied on the root folder, so you hacked something to get there."
Me: "No, I knew the name of the file so I was able to access it just like any other file."
Dev: "That is only because you have admin permissions. Normal people wouldn't have access"
Me: "I could access it from my home computer"
Dev:"BECAUSE YOU HAVE ADMIN PERMISSIONS!"
Me: "On my personal laptop where I never had to login?"
VP: "What? You mean ...no....please tell me I heard that wrong."
Dev: "No..no...its secure....no one can access that file."
<click..click>
VP: "Hmmm...I can see the system administration password right here. This is unacceptable."
Dev: "Only because your an admin too."
VP: "I'll head home over lunch and try this out on my laptop...oh wait...I left it on...I can remote into it from here"
<click..click..click..click>
VP: "OMG...there it is. That account has access to everything."
<in an almost panic>
Dev: "Only because it's you...you are an admin...that's what I'm trying to say."
Me: "That is not how our public web site works."
VP: "Thank you, but Adam and I need to discuss the next course of action. You two may go."
<Adam is her boss>

Not even 5 minutes later a company wide email was sent from Adam..

"I would like to thank <Dev> for finding and fixing the security flaw that was exposed on our site. She did a great job in securing our customer data and a great asset to our team. If you see <Dev> in the hallway, be sure to give her a big thank you!"

The "fix"? She moved the text file from the root to the bin directory, where technically, the file was no longer publicly visible.

That 'pattern' was used heavily until she was promoted to upper management and the younger webdev bucks (and does) felt storing admin-level passwords was unethical and found more secure ways to authenticate.