[nitrogen]

There was also the problem of Microsoft changing important Windows APIs just before release. They obviously knew about their own changes, so they updated Word accordingly, while WordPerfect had to rewrite parts of their software after whatever version of Windows was released because the APIs didn't match the developer versions they were given by Microsoft.

Reference: http://www.groklaw.net/article.php?story=20080317124445613 (search for "API documentation").