As with any show that runs into extreme longevity, The Simpsons will forever be shackled by a need for fidelity to the past that in no way stands in the way of unique, original storylines to keep fans happy. But with very nearly 550 episodes under their belts, that attention to detail in writing can inevitably suffer, and continuity errors that conflict established facts and storylines in the canon can and do appear, leading to unquantifiable rage from fans, and accusations of sloppiness. To combat the inevitably disproportionate response to the appearance of plot-holes and missed continuity threads, later episodes have simply ignored canonical details for a laugh, but if you look hard enough, there are plenty of examples that suggest that someone in the bowels of Simpsons HQ wasn't really paying enough attention, and let some glaring issues get in. These aren't always ruinous, by any means - after all, this is a cartoon played for laughs - but the devil is in the detail, and as with anything that encourages such familiarity with characters, and depends on the audience knowing and loving them, it is annoying to see something clumsily missed. Behold, the most affectionate list of mistakes you're ever likely to find on the internet...
20. Homer Got His Job Several Different Ways
Despite being horribly under-qualified, massively uncommitted, and basically a walking disaster area, Homer has shown unfeasible durability in terms of his job at the Nuclear Power Planet, despite multiple sackings and apparently just turning up when he wants to, to do very little at all. You have to wonder how he ever got the job in the first place, and luckily the show has the answer. Or, several answers in fact. In "Homer's Enemy", when Frank Grimes asks Carl and Lenny how Homer got the job, they say he simply turned up on the opening of the plant and got hired, which contradicts two other episodes. In "The Blunder Years", we learn about the tragic fate of Smithers' father, which establishes that the Nuclear power plant was clearly open when Homer was young, and also when it is revealed that he failed his initial interview and had to threaten Mr Burns in order to get hired at the second time of asking when Bart is conceived.
19. Lisa Has A Wandering IQ
We might definitely know that Lisa is a genius, as has been established almost relentlessly since her earliest appearances, but quite how much of a genius seems to be a matter for debate. In "Homer's Enemy", her IQ is revealed by Homer to be 156, despite the fact that Lisa herself has come up with two alternatives: 167 in "They Saved Lisa's Brain" and 159 in "Smart and Smarter".