Debugging tools rely on compiler-generated metadata to present a source-language view, but current compilers often throw away or corrupt debugging information in optimised programs. Attempts to test debugging information are confounded by ad-hoc limitations of the debug info formats and a lack of clarity on whether the compiler or the format is to blame for any given loss. Adopting the "residual program" conceptual view of debug info, we conduct a study of the quality of debugging information in respect of the <i>source-level dynamic call trees</i> it can recover. We compare the trees recovered from optimised and unoptimised versions of the same program, producing a classification of the observed divergences. For each class, we analyse whether format or compiler is to blame and identify specific ways to address these defects. We also validate our classification across a larger collection of well-known codebases.