In the episode where Bart wants to become a cop the police and snake go through the alley but when snake drives his car back at Bart he can't fit through
They explain the joke for this when Chief Wiggam says something to the effect of, "Didn't this alley used to be bigger?" The writers just couldn't think any other way to save Bart so they did that, and it was funny. But not a slip-up at all.
And hey, the Simpson’s are a comedy. The line "...lucky this ally got so narrow in the middle", even though the cops and Bart just came through it, was just for you to think, "What the? awe well... funny anyway." Law of matter and physics don't matter in a cartoon, and this 'slip-up' was intentional.