Currently I make a lot of SharePoint projects and in one case a I faced very interesting problem. One of the project requirement was to delay workflow for specific amount of time, so for me it was obvious to use Delay activity. In fact for some states I needed three of them. Piece of cake, but when I compiled the project I get strange error:
“Only the first child activity of ‘EventDrivenActivity’ ‘eventDrivenActivity1’ can implement the ‘System.Workflow.Activities.IEventActivity’ interface”
What’s going on? There’re a lot code and activities, so it was difficult to find out what’s the source of problem. Finally I disabled Delay activates and it works. So what’s the problem. On the network problem is solved in different way, but the main explanation can be found at MSDN. You don’t choice and you have to make some work around.l I have to switch to sequential workflow.
By the way the new workflow platform allow to make such implementation… I have to check it 
