One of my Microsoft books (Code Complete I think?) mentions that originally the Mac and Windows codebases were actually different for Excel. The Mac team would port the majority of the code from the Windows port and add whatever was necessary to get it going.

It then goes on to give a good example of feature creep. The original spec to add colour to a cell stated that the colour tag should be at the start only and was invalid everywhere else. The colour parser however happened to allow the colour tag to be anywhere. They then extended the spec as it was "free feature". This turned out to be a bad thing as the formula parser didn't expect colour tags in the middle of a formula and crapped out in subtle ways.

The author of book ported the feature from the Windows branch of the code over to the Mac branch. When testing he came across these bugs and assumed that something was broken in his port. He went back to the Windows port and found the same bugs so in the end they had to insert a bunch of kludges to handle it as it was now part of the spec.