mistakes by developers when creating a... whatever

1. Not communicating with sysadmins. You want to discuss technical issues with your sysadmins early on so they can figure out what kind of hardware will be needed to handle the load, and they can propose methods that work within the current infrastructure to do what you want. I know you like working with Storables, but loading them off an NFS filer vs getting just the data you want from a MySQL cluster is an easy pick for a sysadmin. And can we say "application servers" (i.e. FastCGI)?
   
   
2. Picking the latest tech, or any tech based on interest or novelty or perceived design gains. Do the opposite: start with the oldest tech and go up as you look at your requirements. The reason is again with the system in mind: does the tech you want to use scale well? Does it have a long history as a stable production-quality system? Does it support all the methods you'll need to work with it, in Dev, QA and Production? Is there any straightforward deployment model that works well with it? Most importantly: Do you know it backwards and forwards and can you actually debug it once it breaks on the live site?
   
   
3. Not keeping security in mind at design-time. I still meet web app developers who don't know what XSS or SQL injection is. You need to take this seriously as a hosed website can cost you your job.
   
   
4. Not using automated tests for your code. You need to know when an expected result fails so it doesn't find its way into your site. Not QAing or *laugh* not syntax checking your code before pushing it also falls into this category. Test, test, test.
   
   
5. Writing crappy code. Oh yes - I went there. There's nothing more annoying than doing a code review 2 years in and looking at all the bloated, slow, confusing, undocumented, unreliable, crappy code from the early guys who just wanted to get the site off the ground. There's always going to be bit rot and nobody's code is perfect. Just try your best not to cut corners. There is never a good time to rewrite so try to make it last the test of time. A good example is some of the backend code i've seen in some sites: the same app being used for over 10 years without a single modification and never breaking once. Also good to keep in mind is portability. When the big boss says it's time to run your code on Operating System X on Architecture Z, what'll it take to get it running? (hint: you'll probably be doing that work on the weekends for no extra pay)