Whenever it's time for you to upgrade modules on your site, you should have a checklist of the steps involved. One step I've recently added to my checklist is to clear the site cache just prior to uploading the updated module files.

I learned this lesson with a recent Views upgrade. While working on my dev server (#1 on my list!) I found that if I didn't clear the cache prior to installing the updated Views module, I was faced with the dreaded White Screen of Death (WSOD to those of us familiar with it). After a few minutes of my time in the Views issue queue, I found other people with a similar issue and the result was consistently some type of cache-clearing activity. Once I reinstalled the old versions, I cleared the cache, reinstalled the new versions and all was great.

For the record, here's my module upgrade checklist:

1. Perform the upgrade first on a machine other than your live server
2. Login as User 1
3. Backup the database and files on your live server (I use the Backup and Migrate module for the database backup)
4. Put site into maintenance mode (admin/settings/site-maintenance)
5. Clear the cache (either from admin/settings/performance or via the Admin Menu.
6. Delete the modules you're going to upgrade (don't just overwrite them with the new modules, delete them first).
7. Upload the new modules
8. Go to the admin/reports/update page to verify the new modules are installed
9. Go to update.php and run any database updates
10. Go to the admin/reports/status page to make sure you're all green-checkmark happy
11. Test your site
12. Take your site out of maintenance mode.

Did I miss anything? Do you have any additional steps in your module upgrade checklist?