I have heard many times from Rabbinim and in lectures that to pick and choose rulings is forbidden. I would like a halachic source for this. I can understand that one should be consistent, but if one follows the Aruch Hashuclhan in one matter and decides to pasken like the Mishna Brurah in another matter lekula in both cases is he doing something halachic wrong? I am not talking haskafickly only halachic.
I asked a similar question to a prominent Israeli Mechaber-Sefarim.
He said there is no problem of following one Rabbi in one topic or sub-topic (such as the Brachah on cake) and another Rabbi in another topic or sub-topic (such as shinui makom - a change of place in regards to Brachos).
If you follow two different Poskim with-in the same sub-topic, you will likely be acting in a contradictory manner. That is why one may not always pick the most lenient approach in every case.