This code triggers the following error: "System.UnexpectedException: User defined type found in unsupported code path" when a debug console is open. To replicate, create this as a class, and enable the "Override log filters" setting on the class within Develop -> Classes

Salesforce bug

public class TestSalesforceBug {

	enum Day {
		MON, TUE, WED, THU, FRI
	}
	
	@IsTest static void testMe(){
		Set<Day> d = new Set<Day>{
			Day.MON, Day.TUE, Day.WED
		};
		
		if( d.contains( Day.MON )){
			System.debug('Has Monday');
		}
	}
}