Here is a little gem that I have come across recently...  I have only confirmed this in BizTalk 2006 R2.

If you create a receive shape with a filter expression of (BTS.Operation=="somevalue") and your namespace contains BTS. then you will get a build failure similar to:

The type or namespace name 'Operation' does not exist in the namespace 'Spike.BTS' (are you missing an assembly reference?)

It appears that during compilation it is trying to resolve the BTS.Operation in the project assembly.  I was surprised by this one and have created several projects in the past with BTS. in the namespace without encountering an issue.

To illustrate, I created a project named Spike.BTS.WeirdSubscriptionIssue

If a receive shape is created with a filter expression referencing any BTS.* class, you will not be able to compile. 

Where

Strange...

Anyone else encountered this before?