I will mention some points below that some website developer do in the real world and they don't know it's bad....
You don't test your code on the local host but you believe all testing will be executed on the production site.
You gave anonymous user a PHP filter option in the comment field, or allowing full HTML option in comment field to see if people can actually hack your website via the comment section.
You don't update your website development experience, but when you like to do,you visit a 2006 blog to read stuffs you read when you were learning website design and development....
After uploading your files website to your remote server, you forgot to change the permissions from 777...
After you claimed you have a 15 years experience in website design and development...You don't know the tricks of the trade...My friend..You don't need to memorize the whole HTML language or PHP syntax ...there is a shortcut to most of it....coming up on this blog in few weeks
You still use table in designing web-pages instead of simple CSS..
Your favorite way to ‘integrate’ 3rd party code is to use PHP includes from external URL.
You believe you can fix errors without asking your friends because you think its embarrassing, and you don't know places like IRC or have a developer channel, and you joined all non-useful groups on Facebook
If you don't know about responsive design till now and you don't care to find out.
You insert unnecessary animations on the website making it look too colorful...
You use Marquee text in browsers now, especially if you design website for people of ages 35-60 knowing fully well they use Internet explorer.
You still want to get all users information like we are in the 80s, i close any registration that exceeds a page unless i am winning some $$$$ after the whole registration....learn to summarize registration, instead make them login with social media links like Google, Facebook, yahoo and twitter Authentication.
Having a client website play some Annoying song/video after loading...even if you designed a music website, ask for user's permission. I will curse you if i sneak to check a DJ website because i wanted to invite him/her to a party only to allow my manager ask me to go home if i want to party or work because the website was auto playing some unknown music
You created a blog site or a website with archive and no search box....sometimes i ask myself if the web-designer wants to eat the search box....smiley. for crying out loud, use Google search box at least....hahahaha
You claim you don't like wire framing before developing your website, this is like saying i want to build my house but i don't need a plan...they are in my head....heheheh....You are drunk...
Writing a long lines of big grammars to describe something on your website when you could have used a picture, always remember this " A picture is worth thousand words"
Flash is really awesome on a website, sincerely i love it so much like i love a girl, but i wont use it on a website that the client doesn't specifically require it..."No hate to flash developers"... My point here exactly is don't use flash on Navigation...that is a No-No please.
Most web developers don't do this, but i still want to point it out...if you don't have a contact information page on a beautiful website...that is not cool at all, and even when you have a contact page, place it on the footer too, so user can navigate to it from anywhere they are browsing on your page.
Most developers don't do this because they believe its irrelevant, but it is...A favicon...read about Favicon here.
Not testing out your websites on different browsers like Google chrome, opera, Firefox, and internet explorer. Also if you have your tablet device and mobile, check to know your website is compatible with the tablet and mobile friendly too