When doing a scoreboard operation such as:

scoreboard players operation @e[type=Pig,name=Piggy1] obj > @e[type=Pig] obj

It will return an error

The entity UUID provided is in an invalid format

when there is more than one Pig around.
This happens with most every entity and every operation.

Edit:
It doesn't happen on players

Edit:
I did more testing and it gives an error when:
1) The first selector is a non-player entity
2) More than one entities of second selector are targeted (which includes players)