You must be surprised how can this be true. Yes this is true statement and I have seen that situation.
One of the application-admin came to me with hung application. He asked me to take a look in SQL Server. Looking into database server, log drive is filled. As log drive filled and cannot grow, applications user will get all kind of error in their applications. We always assign enough space assigned to log drive not to get log out of space issue. After troubleshooting it was just on one open transaction never commited/rollback.
In SQL Server Transaction log get flushed and reused after transaction has been commited/rollbacked. But when transaction keep open for long time say days or week, all the logs created from different transactions after that uncommited/un-rollbacked transactions never get truncated to reuse log space. So SQL Server log keep growing. If the database server is highly transactional, log space hits the wall soon.
In this case, only way to resolve the issue is, kill that session spid.
after killing that SPID, voila!!! you can recover all the log space.