A "sign error" refers to a mistake made while using positive (+) and negative (-) signs in a calculation or equation. It happens when you accidentally use the wrong sign, resulting in a wrong answer. Full definition
(At the position recorded by Messier, which also found its way into John Herschel's GC as GC 1594 and, consequently, into Dreyer's NGC as NGC 2478, no cluster is found, so that this object was missed, until T.F. Morris, in 1959, identified it correctly as Herschel's cluster H VIII.38 (NGC 2422), and realised that Messier had done a simple sign error in RA difference when reducing the positional data.) (maa.clell.de)
Michael, I believe you have a minus sign error in one of your equations, reproduced below. (carnegieendowment.org)
Now in fairness to Spencer, he is only responsible for a combination of algebraic and sign errors that led him to a cooling trend from satellite data that correctly analyzed, actually indicated warming. (realclimate.org)