I should begin by disclaiming that all predictions of the future in Iran are subject to high error. But I ran across a few bits of information that, especially taken together, are a bit ominous. There seem to be indications of some sort of crackdown being prepared for Friday, and other indications that the reformist politicians are worried about one and trying to sidestep it.
TehranBureau, which is based outside Iran but has generally been pretty good, reports "trustworthy sources" saying that "many Sepaah commanders [Sepaph is IRGC] have been arrested, because they are opposed to what is going on and in particular to the plan for tomorrow". An update clarifies: "Apparently, the plan is to create chaos and bloody confrontation between Basij and Karroubi and Mousavi demonstrators, in order to justify hard crack down and have Khamenei announce the end of "soft" confrontation in the Friday prayers."
Other sources (mostly low-profile Iranian twitter users, with accounts predating the current turmoil and not very good anonymity, so I'm not going to link them) are reporting that Karoubi and Mousavi are canceling their Friday demonstrations, postponing them to Saturday, without explanation; one source links this statement from Karoubi in Farsi; anyone able to confirm it says that? That would seem to indicate Mousavi and Karoubi have heard enough similar rumors, and take them seriously enough, to try to sidestep something planned for Friday.
It should go without saying that hopefully I'm wrong, and things will go as well as can be hoped for under the circumstances. The weakest link in this speculation seems to be the TehranBureau report, which no other sources have confirmed. The canceling of planned Friday rallies, which does seem confirmed, seems to indicate something is up, though.