But it turned out that using four substitutes was all above board on this occasion for Gary McSheffrey.
Plymouth informed referee Martin Coy they were using a concussion substitute at half time when withdrawing Adam Randell, who had suffered a head injury during the first half.
Doing so meant Argyle still had three standard substitutions remaining - and it also handed Rovers the possibility of an extra change should they wish to use it.
And McSheffrey admitted he was not aware of the technicality until being informed of it at the break.
He said: “I got told at half time they’d made a concussion sub which allowed us to make four subs in the second half.
“I wasn’t really aware of that but that was the reason behind it.”
McSheffrey introduced Kieran Agard, Ben Jackson, Charlie Seaman and Adam Clayton off the bench.