Hashtables keyed off the address of objects would be an example.

On multiple runs, malloc gives out different addresses (thanks to threads or security concerns) which means things end up in different slots in the table. Then you iterate through it in memory order and you're seeing objects in non-deterministic order, which you do things with.

Embedding file paths / timestamps / git shas and similar was popular for a while too and unhelpful for reproducible builds.