What conversion/rewrite have you been involved with that failed? What where the languages and framework involved in the process? How large was the software in question? Finally what is the top one or two thing you learned from being involved with the process.
This is related to this question.
One of my favorites was a fixed price project developing a server app on Sun OS 3 that was eventually required to run on an old Data General machine running some (at the time) current version of Unix.
Now, we knew ahead of time it would be painful porting the C code to the Data General because it had 9 bit bytes / 36 bit words but didn't realize how ... lacking... the Unix implementation would be.
It was approximately 5 hours from the start of the porting task when the wise executive decision was made to simply buy the customer a shiny new Sun box.