I want to use Email This Issue to create issues with different request type based on the appearance of certain keywords on the email subject.
I set two if statements on the incoming mail handler:
If the subject contains 'test', then create issue with request type A in Project X
If the subject contains 'demo', then create issue with request type B in Project X
If the subject contains 'sample', then create issue with request type C in Project X

The result is that:
1) If the email subject contains 'test', issue with request type A in Project X will be created without problem
2) if the email subject contains 'demo', the issue failed to create
3) if the email subject contains 'sample', the issue failed to create

It would fail to create the issue if the first statement failed and the successful statement is creating an issue in the same project as the failed statement (i.e. in this case, both are creating at project X).
It would create the issue successfully if the successful statement is the first statement, or if the issue is created in a different project than the failed statement.