The 5 Most stupid (and common) errors of SharePoint developers

If you are a seasoned developer you'll probably agree, if you are not so experienced you'd have probably suffered at lest one on the last week and lost an hour before figuring out what was happening.

Updating the wrong web
You are changing something, the code works, no exception is triggered but you go to the SharePoint site and nothing has changed... Check the web you are pointing at, you might be updating the wrong one.

Wrong field name
Your CAML query is fine but you get an exception saying that the list's fields are wrong. Go to the list definition and to the field definition, there in the URL you have the field internal name as SharePoint expects it, yeah you got it wrong. I like to have them as constants somewhere, spell them one properly and let the intellisense remind you of the name for the rest of the project's life.

Wrong list
The item you are trying to create doesn't appear in the list or it complains about the field names and they are right... Check that you are connected to the right list. (I also have constants for the list names)

Wrong DLL
You change something in your code and still you get the same error. Depending on what you are doing you might need to deploy the DLL to the GAC or the BIN folder, the right DLL, and then reset the right service, sometimes it's not the IIS. Try changing a string or something very obvious to spot so you are certain the code you are running is the code you want to run.

My web services won't connect to the server
You need to create a clientaccesspolicy.xml file or a crossdomain.xml and put in in the web application's inetpub folder, so simple and yet so annoying.

[BONUS] The changes I am making do nor appear in SharePoint
Right list, right web, right fields, right everything and yet nothing happens when I change the object... You forgot to update it. And this goes for a lot of different items in SharePoint, not only list items. In doubt I always try to add a .Update after modifiying. Beware, the updates are usually slow and should be kept to a minimum.