I maintain the open source CAM::PDF Perl library in my free time. This library, originally authored by me at Clotho Advanced Media starting in 2002, is a high-performance low-level PDF editing tool. It doesn’t have support for sophisticated authoring tasks (see PDF::API2 for that!) but it is good for utility work like concatenating two PDFs together or deleting pages from a document or encrypting a PDF.
I just fixed a bug where appending a big PDF to a small one (“big” in terms of number of internal objects, which correlates with page count or byte count but is subtly different) often went wrong because I generated object ID numbers by simply incrementing a counter in the small PDF. Sometimes, that counter matched IDs in the bigger PDF, but those IDs are supposed to be unique per document, so things went badly. This bug is now fixed by simply taking the max ID of the two docs as the new counter value before incrementing to make a new ID number.
I never stumbled on the bug in my own work because I always appended (or prepended) the smaller doc to the larger one for performance reasons. CAM::PDF v1.54 is on it’s way to CPAN as I type this.
I’m grateful to Charlie Katz of the Harvard-Smithsonian Center for Astrophysics for providing me with a simple test case that exhibits the problem!