Package becomes unrecognized in Eclipse

Error:
Saving a Java source file in Eclipse causes the errors "[class] cannot be resolved to a type" and "The import org.oasis_open.docs.[whatever] cannot be resolved" for classes in a package that was previously recognized.

Solution:
Open the context menu for the affected package in Package Explorer, select Build Path > Configure Build Path... and go to the "Order and Export" tab. Make sure custom classes are at the top, third-party files are in the middle and the JRE is at the bottom of the list. Select OK and refresh the affected files/packages.