Early episodes had Roger gradually learn to use disguises to go out into the world; but in later seasons we see that he had actually been using disguises for decades before he met the Smiths.
In the Smith house there's a window at the stair landing that appears through most of season one, but is never seen again in subsequent seasons.
When Sllort transforms into a Troll, the Troll's bust size goes from large to small between scenes.
The Engineer says there are 49 possible Jenga moves, when there aren't enough blocks in Roger's Jenga tower for that to be true.
Steve's Paranormal Magazine has a misprint on the cover: "Yetti" instead of "Yeti".
The newsreporter says "We interrupt this silent film", but there has been none of the incidental music that is used with silent films.
MDMA was not invented by Roger the Alien, it was a Pharmaceutical Company's side-discovery during the fine-tuning of what became Antidepressants.
When the family are in The Diving Bell retrieving part of Roger from the Marianas Trench, Rogu looks much clearer than the rest of the family through the window.
Stan says Tommy has a DVD rack full of DVDs when it is clearly a CD rack full of CDs. CD boxes are squarer.
When Jimmy Boombox is teleported from his wheelchair into Mind Quad's sidecar, Jimmy Boombox no longer has his Boombox.
Rogu doesn't have Ear Slits like Roger does.
Francine says Steve is allergic to bees, though he has been a Beekeeper in a previous episode.
When Roger goes through the Blorpher, the blood spatter is red when Roger is an Alien and does not have red blood.
Snot develops an affection for the Turtle "Bartleby" despite having previously been turned into a Turtle during a Time-Travel Accident.