5. Forgetting that incremental costs can be subtle.  One thing ABC teaches us is that overhead costs may vary with drivers other than volume.  For example, if you have more parts in a device, you are likely to have more costs (e.g., due to design, handling and storage of these parts).

6. Assuming fixed costs cannot be incremental (and thus cannot be controlled).   Fixed costs are incremental in many decision contexts.  Control of fixed costs is crucial in many (if not most) businesses, and you do not have to unitize them to control them.

7. Treating sunk costs as incremental costs.  If you've already spent the money, you cannot get it back.  Of course, you can allocate it to products, customers, whatever.  For example, consider depreciation on equipment that was purchased and paid for in the past.