When One-Eye rides up to Rollo and Jarl Borg, the blood splattered on both Rollo and Jarl's chests change from before One-Eye rode up.
During Rollo's trial the fjord changes radically in appearance between certain shots, appearing as if the VFX-artists has neglected to add the Norwegian background to the Irish set.
When Ragnar chases after the departing Lagertha, he leaps off his horse, clearly showing a modern stirrup. In 796, stirrups were just becoming known in Scandinavia and northwestern Europe, based on archaeological evidence. It would be convenient for the show, but rare to find them in a relatively unsophisticated location.