Think about the last time you had the actual opportunity or reason to multi task. You were doing something, then you ran out of things you could do to move forward so you switched over to something else.

i see it all of the time with software development. You know you need to write a user interface, so you start putting things on the web page, you start adding stuff, and it suddenly occurs to you that you don't know an answer to a question. "I wonder if they want blue or red for this?"

"I guess I better call the business analyst/customer etc"

so you pick up the phone and you get their voicemail. "Damn!", you think.

You leave a voicemail message and you move on to something else.

Now you've spent a good 10min getting ramped up into the task at hand and you're bopping along. Your phone rings. "Damn, i was on a roll!", you think.

So now you know they want Red, and you now either go back to making that widget Red or you continue doing your second task.

Is there no way around this dilemma?

Well, you could make an assumption and go with red. Unfortunately, if you forget to ask them then you're going to get a defect though if they wanted blue. I guess you could risk it. Lots of us do this... subconsciously. 

If you do that then you're wasting both your time and the testers time. These wastes are task switching and rework.