In the world of superhero comics, it's pretty safe to say that readers have become pretty well-accustomed to crossovers. In the big shared universes at Marvel and DC Comics, characters show up regularly in each other's books all the time, and even if they're keeping to themselves, there's always the big, universe-spanning event comics that are rolling out like clockwork to bundle them all together for your reading enjoyment -- or for your reading, at least.
In the world of newspaper strips, however, that sort of thing is much more rare. Sure, you occasionally get stuff like Tom Batiuk arranging for a shockingly boring cross-time comic book sale in Crankshaft and Funky Winkerbean, but even that's pretty small and confined to one character.
As a result, it's always notable when the newspaper characters start jumping into each other's strips. Especially when it's two-fisted cop Dick Tracy gearing up to rescue Little Orphan Annie from the clutches of a murdering terrorist known only as "The Butcher of the Balkans," a thing that is actually happening iny our newspaper right now.