4 parshiyos

R' Akiva Eiger (O.C. 685) writes that if the tzibur forgot that it was parshas shekalim and didn't read the parsha, they should take out a sefer once they remember and call up one person and read the parsha.  Others disagree, as we never find a case of a stand alone kri'as haTorah where one person gets called up for an aliya.

Yesh lachkor: Is the takanah of parshas shekalim just a din in kri'as haTorah, defining the subject matter to read that Shabbos, or is it an independent takanah?  

The Emek Bracha raises the same issue with respect to parsha zachor.  He proves that reading the parsha is not a mitzvah in its own right of zechiras Amalek, but rather is just the subject matter Chazal chose for the krias haTorah that day. The Rambam does not mention the kriah in Hil Melachim (5:5) where he discusses the mitzvah of mechi'as and zechiras Amalek, but rather in Hil Tefilah (13:20), where he discusses what parshiyos to we read on the Yamim Tovim.  

Nafka minah: whether one must have kavanah for the mitzvah of zechiras Amalek when reading the parsha, or is it just a regular kriah like any other?

This seems to be the debate between R' Akiva Eiger and the Birkei Yosef, who disagrees.  R' Akiva Eiger understood that reading shekalim is an independent takanah, so if you missed reading it as maftir, just take out a sefer and have someone read it whenever they can.  The other side of the argument held that parshas shekalim is just the subject matter of kri'as haTorah, not a new din, and therefore can only be done in context of the normal rules of kri'ah.