Every once in a while (when I use a new dev environment) I hit this error:

Saving Changes in not permitted. The changes you have made require the following tables to be dropped and re-created. You have either made changes to a table that can’t be re-created or enabled the option Prevent saving changes that require the table to be re-created

Each time I forget how to fix this, so by posting here I know I'll never forget. Check Pinal Dave's blog post on this error (hint: it's in the Option menu). Thanks Pinal!