1 {scene: a prison cell in Berlin}
1 Monty: So, dad. Pop is dead. And here we are, stuck in a cell in Berlin again, surrounded by the Nazi army.
2 Prof. Jones: Things aren't quite as bad as they seem, Junior.
3 Monty: Oh, you see some good news in all this?
4 Prof. Jones: I just remembered, I lent my hip flask to Sallah, not Pop. So it wasn't destroyed after all!

As an astute reader pointed out after #565, Professor Jones was incorrect in saying that Pop Jones had his hip flask, as he'd actually lent it to Sallah way back in #508. So this strip reintegrates the continuity of the story.

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2013-09-12 Rerun commentary: It's always good to have an absent-minded character. It makes these sorts of in-character explanations of continuity gaffes feasible.

What we have here is technically known (at least where "technically" refers to what TV Tropes calls it) as a Revision. It's a fix for a continuity error which is fully explained by later events without needing to reverse, change, or ignore what was previously considered to be an error. It basically acknowledges that there was an out-of-setting continuity error, but explains it within the fictional setting.