NUnit Test Debugging

I'm working on a Geometry library. There are 200+ unit tests.

There's a particularly stubborn test right now that fails whenever I select Run all, but the test passes if I run that test individually, or use the debugger on it. I can't say whether the test ought to pass or fail right now, but it definitely shouldn't do both. There may be an issue with the method as written, but something also is wrong with the way I'm using NUnit or the framework itself.

Now some notes about the geometry library:

The objects are immutable (Or atleast are intended to be)

The tests have no shared objects between them.

So my Question: What are the possible causes for this odd behavior?