DIFFER combines elements from differential, regression, and fuzz testing to help users find bugs in programs that have been altered by software rewriting, debloating, and hardening tools. We used DIFFER to evaluate 10 software debloating tools, and it discovered debloating failures or soundness violations in 71% of the transformed programs produced by these tools. While DIFFER is application-agnostic, we originally built this tool to help us find bugs in programs that have had unnecessary features removed with a debloating tool (e.g., Carve, Trimmer, Razor).