I once reverse engineered a spreadsheet that had cost the best part of a million pounds to develop - I made the relevant manager very unhappy by pointing out that significant chunks of the beast did absolutely nothing.
I did come to really like the "Trace Dependents" and "Trace Precedents" features in Excel! :-)
Yeah... the worst part of spreadsheet is that it's hard to document their structure and purpose. You may try to explain it with cell comments and blocks of color, but there's no easy way to signal the entry point or see an overview of how the different parts work together.
At least they are functional reactive, so it's easy to trace the evaluation of expressions.
I did come to really like the "Trace Dependents" and "Trace Precedents" features in Excel! :-)