When Marmaduke has the cone around his neck, his collar comes off along with it when he pulls it off with the fence. Then it's shown on his neck again as he walks away.
The plane taking the Winslows to California has winglets when shown taking off. The plane shown landing does not.
Phil is driving along the coast with Marmaduke in the back seat. The shoulder belt section of a three-point seat belt is clearly around Phil's shoulder and chest. When Marmaduke complains of a bee, Phil turns to comfort him and the belt is gone. It reappears in the following shot.
Bosco and his gang constantly refer to themselves as pedigrees, and Marmaduke and his friends as "mutts." A mutt is a mongrel, a crossbred dog, but Marmaduke and his friends are all pure breeds. He is a Great Dane, and his friends are an Australian shepherd, a Dachshund and a Chinese crested - they are all just as much pedigrees as Bosco and his cronies.