Chapter 5/Time off Management Error page#106

I kept getting the following error when running my Workflow.

" An error has occurred in IT Dept Time Off Request Workflow. "
(note:"IT Dept Time Off Request" is merely the name for My TOR Request list/templated)

PROBLEM:
I finally traced the error to the multiple(2)
"then Update item in MasterTimeBank" action/statements in the
step named: "Request Approved: Decrement Requested Hours...".

Even though they are supposedly Sequential Actions(then), they seem to be conflicting. I deleted the second statement & my Workflow completed successfully.

SOLUTION:
I combined the TWO Update Actions into one statement & my workflows completed without error.

QUESTION/ADVICE:
1) Should the 2 actions run sequential anyway ? any ideas?

2) Whenever testing, I'd manually start it without any initial parameters;
AND I'd get an error page; ?What controls the response/page after a
manual Workflow start with no initial parameters ?