I've got an interesting question/statement about event handlers:

Tasks fire the same EH at the same time. My understanding is all EHs fire at the same time (Parallel).

If I understand the question correctly, the package has an event handler that can handle multiple events, and these events fire at about the same time. What happens?

Well, I did not work much with event handlers, so let's experiment and test how it works. This is really simple. I've create a package with two dummy script tasks A and B (not connected with any precedence constraints) that fail (by returning failure) and OnError event handler on the Package. The event handler contains a simple script task that prints the name of the event source:

MsgBox(Dts.Variables("SourceName").Value, MsgBoxStyle.Information, "Event Handler")

What will be the result of executing this? Will we get two message boxes at the same time, or sequentially two message boxes?

The answer is - two message boxes popup sequentially one after another, but never at the same time. Ssometimes A is the first, sometimes B. The event handler (like any other task) is not reentrable and only a single instance of an event handler can be running at a time, thus the second event has to wait for the event handler to finish processing the first event before processing the next one.