Joke-stepping is particularly noticeable in certain comics, where attempts to cram several related punchlines into a short space such as a final panel are more obvious than in filmed media.
In some usage, to step on a joke means to ruin it by deflating it or explaining it. This is more accurately called 'killing the joke' which can be a deliberate form of anti-humor. Stepping on the joke usually isn't.
Did you see that Jay Leno monologue? Making all those lame comments after his punchline? He stepped on that joke so hard he practically curb-stomped it.
To step on the joke:
It was like he was going, "Kiss the curb, you joke!" Heh. Hmm. Yeah. Whoo.
*Applause sign flickers*