Teemu Talja 
I notice that I order usability test tasks usually in the following way:
- Participant’s own tasks.
“What did you do last when you used this? […] Let’s try this now.”
- Structured tasks.
“Your girlfriend has birthday tomorrow. You want to buy movie tickets for you both. How would you proceed?”
- Review tasks.
“Please have a look at this [part or feature of system], and feel free to tell what you think.”
Such an order minimizes priming effects caused by previous tasks. Priming effect means that what you have done a moment ago affects how you solve problems and act now.
It is good to prepare ten – twenty tasks for a test. An experienced, fast-paced user may be able to accomplish fifteen – twenty tasks, whereas an inexperienced may do seven or eight.