A friend of mine went through this as well. On a driving trip from Arizona to Banff, Canada he stayed behind in Montana while I went into Canada because his green card was still processing.
As stated above, leaving the country without prior approval will be considered abandonment of the green card application. Although you may be able to reenter if you still have some other valid document, if they connect the dots and realize you left the country, they can cancel your green card application and force you to start over.
Seriously, USCIS is the worst government agency to deal with.