According to the view that the 13 Divine Attributes ("Hashem, Hashem, Keil Rachum...") are considered a davar shebikedushah and therefore require a minyan for their recital, an individual doesn't recite them if he's praying alone (unless he says them as if reading from the Torah, i.e., with cantillation).
What happens if one is indeed praying as part of a minyan, but is a little behind? (And is there any difference in this regard between the various prayers in which the Attributes are recited - tachanun, selichos, etc.)?