Richard de Vries explains on Slashdot:
I work for a small Dutch company that uses Google Apps. This means that we can share documents with users within our domain (www.deondernemers.nl), as well as @gmail.com accounts or other Apps-domains. About three weeks ago, we discovered that some fifteen documents and spreadsheets were unintentionally shared with a lot of people, some of whom were outside of our domain. We found out that one of us had been wanting to share these documents with a colleague (within our domain). He selected the documents on the documents list and added one user. Google Docs then shared all these documents with everyone who had access to one of the selected documents.
De Vries reported the problem last month. Google has fixed the problem and notified affected users. Their letter was reproduced on TechCrunch.