11.) The improper conductor fill of conduit bodies.
This problem is most common in service entrance applications but is found in many types of installations. Section 370-16 ( c ) requires that the maximum number of conductors in a conduit body be computed in accordance with 370-16 (b) in order for a conduit body to contain splices, taps or devices and the cubic inch capacity must be marked on the conduit body by the manufacturer. So in most cases the cubic inch capacity/maximum number of conductors is already marked on the fitting by the manufacturer.
Example: a typical 200 ampere 3 phase service has been installed in 2" EMT. There are four, 4/0 aluminum, XHHW, service entrance conductors. A standard 2" threaded LB conduit body has been used to make the entrance into the basement where the panelboard is located. It is likely that the standard 2" LB used in this installation is only rated by the manufacturer for a maximum of three 3/0 conductors even though the 2" EMT is rated for the four, 4/0 XHHW conductors. A 2" "service type" LB, which has a larger cubic inch capacity or a larger size LB with reducing bushings could have been used in this installation.