I just discovered that I was getting twice the number of tests run that I should ve been getting. Discovered it when a test broke and I got two identical test failures. Same test, same everything. Got me quite confused, but managed to narrow it down to a certain test class that was a partial class.
The reason it was a partial class was that I had split a test class in two, just to make it a bit more clean. The class under test had a certain method that required a long range of tests and I thought it would be cleaner to have those in a separate file. But since there was one or two helper methods used I figured I could just make the class partial so both files still had access to those methods.
The test framework is NUnit and the tests was run by using TestDriven.Net. Ran the tests both from inside a single test method (reported two tests passed instead of one), on the class (got twice the number of tests passed) and on the whole test project.
Managed to fix the issue by making the classes not partial and just duplicating those tiny helper methods (might move those to a separate helper class or something later).
Now... why on earth is this happening? I thought partial classes were compiled into a single class? Is this an issue with partial classes in general, NUnit, Test-Driven.net or something completely different?