Anyone with some level of familiarity with
Calvin and Hobbes is probably aware of the story concerning the "missing" cartoon.
Here's a quick link to the two strips for 28 November 1985, just ten days into the strip's run; the alternate strip appeared in half the newspapers running the comic at the time, and the one that appears in the books (including
The Complete Calvin and Hobbes) in the other half. Whilst no official statement was made, the generally accepted theory is that the one not archived was altered out of fear that children might try and imitate Calvin's desire to bathe in a washing machine.
Whilst that is the only case of a strip completely changing, there are five other C&H strips with two different versions. In three cases, these are dialogue changes, and Wikipedia's article on the strip gives the dates for the comics in question but not exactly what's different. So, if like me, you Googled those strips to see what was different about them after reading the Wikipedia article on Calvin and Hobbes... here you go.