I have a kitchen that has three countertop outlets all on the same branch circuit, and no GFCIs as the house was built prior to that code requirement. My plan was to replace the "first" outlet in the chain with a GFCI and have the other two outlets come off the load terminals so that way all three outlets were covered via the usual setup. Other than having to sort out which was the "first" one in the chain, no big problem.
To my surprise once I pulled off the covers, the setup is slightly different: the power from the panel goes to one outlet, and then it has two "forks" that branch to the other two outlets. So one outlet has three 12/2 cables coming in, two of which go to the other outlets. I suspect this is because one of the outlets was installed later and it meant running less cable. It happens that the receptacle that's doing the branching is wired with backstabs only -- three hots and three neutrals adding up to six wires being backstabbed into it. (The wisdom of that I won't question here...)
Looking at the instruction sheet for the Leviton GFCI to see if there were any hints for this sort of a setup, instead it simply says:
DO NOT install the GFCI receptacle in an electrical box containing (a) more than four wires (not including grounding wires) or (b) cables with more than two wires (not including the ground wire). Contact a qualified electrician if either (a) or (b) is true.
So why the "don't do this"? Is this just a "this isn't the easy case and the lawyers didn't want to get sued" or is there some code problem here? My plan was just to pigtail the two loads feeding the other outlets together and attach that to the load terminals of the GFCI. The only particular problems I can think of is either the box just getting too full to work with, or in this case since these are 20A small appliance circuits that a 12 gauge pigtail wire is also required.
Update: So the box appears to be stamped as 16 cubic inches, which means this was far to small even for the existing install. The box does appear deeper than the rest of the kitchen outlets but it's not crazy deep. Ultimately the plan was to replace that box with one to add a switch for a garbage disposal circuit, so I'll probably defer the GFCI install and let the electrician handle that one...and make sure they get an extra deep box.