I had to update a Drupal 7 site recently and needed to change the field keys of a list field to be different values. This wasn't possible from within Drupal as it does a pre-check to make sure that the key doesn't already exist. If it finds any values present in the database with that key then it will reject the change. This is absolutely correct but causes a little bit of an issue when you need to update these values.
When overriding theme functions in Drupal 7 you would normally copy the theme function into the template.php and alter it to suit your needs. This isn't always convenient though, especially if you are trying to abstract functionality into modules and not have code in your theme layer that is reliant on module code. If you add theme override functions to your module files then they won't do anything as Drupal isn't looking for them there and won't pick them up.
Steam wrappers were introduced in Drupal 7 and allow user file locations to be kept in a maintainable way, although I often forget which function to use to translate them. The three wrappers available are public://, private://, and temporary://, which map to the public, private, and temporary files directories respectively. All user files in Drupal are stored in either of these directories and they are referenced in the database as the file wrapper followed by the location of the file.
I am currently using SimpleTest to test a complex multi-step form implementation in Drupal 7. It made sense to do it this way as there are a lot of factors involved that all need to be accounted for and automating what form elements appeared on what page was the most robust solution. In order to test how the form worked I needed to submit to the form once (using a $this->drupalPost() method) and then submit the form again using the same method. The tricky bit here was that when calling the drupalPost() method with a URL it first called drupalGet() on the URL before posting to the form.
Whilst working with Organic Groups today I had the need to load a list of the nodes that a user is connected to. After a bit of looking around in the source code I couldn't find a good solution on how to do this. So after looking around on Google for a bit I just sat down and wrote one.
I recently noticed a strange little issue with Drupal 7 that seemed like either an oversight or a decision I don't agree with. Essentially, when a node is created with a menu item in place the extended flag on the menu will not be set, but the control is also not available on the menu admin page. This means that when you are trying to print out a hierarchical menu structure you need to create the page, go into the menu admin area, access the menu, click on edit to access the menu item and change the setting there.