Oh yes, we're fully XML enabled
I missed one off the Programming by Superstition list:
Myth:
DataSets store their data internally as XML.
Rationale:
Microsoft said so!
When ADO.Net was announced, there was a lot of marketing guff about how it was "based around" XML, and how XML was a "first-class citizen". After all, it was the year 2000, and no CxO worth their salt was going to buy into a solution that wasn't "fully XML enabled". In fact, there is at least one article on MSDN which refers to DataSets being "natively an XML structure", so perhaps Microsoft believed their own hype.
Truth:
DataSets aren't really the key structure here - they don't do much more than group DataTables together. In turn, DataTables group DataColumns together, and DataRows act as a "horizontal slice" into a group of DataColumns. Going one level further, a DataColumn is just a wrapper around one of a number of classes inheriting from DataStorage (BooleanStorage, ByteStorage, CharStorage and so on), and those are just strongly-typed arrays with a bit of collection-management logic thrown on top of them. So, no XML. Sorry.
(And just for the record, they would have been feckin stupid to have used XML anyway).