Microsoft Office Word vs. OpenOffice Writer
Open source software can prove to be better than its closed source counterpart. Firefox vs. Internet Explorer being just one very clear example.
But this isn't always the case.
When it comes to word processing, Microsoft Word is the industry standard. The open source equivalent being OpenOffice Writer which is also excellent. It's probably fair to say that for most people, Writer will be able to meet all of their requirements... except compatibility.
Yes it's true that with Writer you can save and open files in the .doc format, but what you see when you open the file may be different to what was saved from Word. The differences range from minor visual layout and formatting to pretty major, and even small discrepancies in positioning can add up to major problems if the document is a long one.
But the biggest and most potentially dangerous issue I've found was in a legal document. One of the clauses referred to another clause by number. However there was no clause with this number, so something was clearly wrong. After opening the document again in Word, this clause did indeed exist - Writer hadn't read the numbering correctly.
Using Writer instead of Word in this case could have been disastrous if the wrong clause was referred to and this had not been noticed. So if you work a lot with important legal documents, open source may not be the way to go.