At my previous job, I was responsible for building an automated-testing framework. Background: We already had a "prototype" which was pretty good and had many tests written for it. It was written in TCL.
For the new project, I was supposed to adapt the prototype to fit a new project. I really disliked TCL, and had just learned Python, so I was clamoring to apply that new knowledge somewhere. So, of course, I decided to re-write the prototype in Python. Since we wanted to do lots of really cool new things (support hierarchical tests better, have all sorts of extra mechanisms to easily create new tests), I justified my decision by saying that writing all this new stuff in TCL would be a nightmare.
In the end, most of the new features (which were way too difficult anyway) were never used. We ended up reimplementing the entire prototype from scratch for almost no reward.
The best part? About a year and a half later, we had to drudge up the old prototype to run some tests on the old project (foreseeable that this would happen). I was worried we'd have to adapt it to some of our new tools (one of the reasons I opted to reimplement in Python). Turns out it took about 2 hours of work.