Combo crossover clashes (CCC) are used fairly often when solving Killers. Two 2-cell cages cannot have the same total if they are in the same nonet, column or row and share a common, or crossover, cell. For this geometry to occur, at least one of the 2-cell cages must be a hidden cage or a split cage.
This example comes from
here, in the simplified step at the end of my walkthrough.
14. 45 rule on C12 3(1+2) innies R1C2 + R9C12 = 1 outie R3C3 + 17, IOU R9C12 cannot be 17 = [89]
14a. R9C12 cannot be [69] (because of overlap clash with R89C1 = 15, step 3)
14b. R9C12 not [69/89] -> no 9 in R9C2, R1C2 = 9 (hidden single in C2), R1C3 = 1, clean-up: no 7 in R5C4
Here I used a combo crossover clash between hidden cage R89C1, which totals 15 from an earlier step, and a split cage (2 innies) R9C12 to show that R9C12 cannot total 15 and therefore cannot be [69].
Combo crossover clashes are useful to eliminate combos but on their own they don't often elimination candidates. In this case I also used an IOU, which is discussed
here, to make a further combo elimination from R9C12 and therefore the key elimination from R9C2.
In this diagram there is also an example of a combo crossover clash between a normal cage R1C23 and a split cage (2 outies) R13C3. R1C23 = 10 -> R3C13 cannot total 10. This can be used with 45 rule on C12 2 innies R9C12 = 2 outies R13C3 + 7 as a slightly harder way to eliminate R9C12 = 17 = [89].
Both of these examples are crossover clashes within a nonet. When they occur within a column or a row, walkthroughs may refer to them as overlap clashes.
Thanks Ed for providing the diagram! Many thanks also for interesting discussions and suggestions on how to improve this post! I've used a fairly loose definition for split cage, to avoid making the definition of CCC too complicated. Ed takes a narrower view of what a split cage is.