Q:

Is it normal for PIs to change projects to unrelated research

As I asked in a previous question, I ended up with a project that is not nearly close to my interests or research expertise, and my PI is pushing me so I publish something in that topic.
But that made me wonder. Is it usual for this to happen in Academia? I'm not talking "Topic was A and ended up with slightly related topic B", No, I'm talking a topic about a whole different field (even) than topic A, let say Topic Omega (Different Journals, Conferences, etc)
I reviewed my CV and the Job posting of almost 2 years ago, and it clearly said the job was on building algorithms for Brain Machine interfaces using Machine Learning. 
However I ended up building electrodes (heavily balanced toward doing electrode characterization) for Brain Signal Capturing, and the paper he wants me to publish is in this topic, that is not nearly related to Machine Learning. Also, nowhere in my CV I even stated that I had nor the expertise nor the experience to deal with these particular field. And at the end of the day, had he advertised it as a heavily experimental postdoc I would not have applied at all.
We are talking Postdoc level here, not a grad student who suddenly switched topics. I just want to know if this is normal, or just a bad situation. 
Knowing that this is normal might reduce my level of frustration, but if it is not, at least I can speak up in the exit interview.

A:

I would say, no, it is not normal for a PI to shift focus so radically.
In the normal run of things, when hiring at the PostDoctoral level (and lower), the PI will have: 

Submitted a funding request with a detailed outline of the research plan.
Have received acceptance of that plan and a budget.
Advertised open positions accordingly to the budget looking for people with the necessary skills.
Hire people with the necessary skills for the duration/budget allowed by the funds.
Supervise execution of the research according to the initial plan.

... pretty much in that order.
Topic shifts would normally occur where funding for a project has run out (which should correspond with the end of the PostDoc contract) and the PI wishes to find funds from other projects to keep the PostDoc employed and to extend the contract. 
Shifts might also happen if the PI is pooling sources of funding together, which can sometimes happen: they might decide to juggle resources between different projects on the fly. This, however, is far from an ideal situation, esp. for managing researchers on temporary contracts.
The questions I would ask in your situation are: in which of the PI's projects you are working on, has it changed recently, where the funding for that project came from, and what the original proposal was. 

If you have moved project, you should ask why. 
If you are in the same project, you should ask why the topic changed so radically all of a sudden.

