Btw, for all those wondering about leaving the country while the app is processing, generally, they'll only grant it for specific reasons - visiting family is not one of them. But if someone gets a note from a doctor saying their relative is sick etc., then they'll grant it, and one can request it to be expedited depending on the date one "must" leave the country.
That is incorrect. I applied for a I-131 - an Advance Parole Document - at the same time I applied for my GC, and received it approximately two months later. This allowed me to leave the country how ever many times I wanted, and for whatever reason.
The only hitch is that when you return to the US, they take you into a side room, filled with other "suspicious" people, e.g. Arabs who they suspect of joining ISIS, and ask review your case, maybe ask a few questions. IME, this process could take from less than 10 minutes, to over an hour, depending how many people there are there and how long you'll have to wait.
Besides for this, there is no problem with leaving the country at all.