A long time ago, when I started building my first video game server for Double Cluepon, my video game company, I did a bad thing. I looked at the AMF library for Perl and Python and decided that Python's looked better. I had always meant to learn Python, and this felt like the perfect opportunity. It had cooperative multitasking (Twisted) and it had an ORM (SQLAlchemy), so along with the messaging format (PyAMF), I had everything I needed to build a server for a Flash MMO (later migrated to AIR).
Let me reiterate my mistake: While under time constraints, I chose to learn a new programming language. I didn't realize my mistake until it was too late.
Being a startup (still), we needed something As Quickly As Possible. So, I cobbled together a marginally-working system from all the examples I could find. Not being familiar with Python's strengths and weaknesses, I tripped over everything, getting more and more frustrated (but still absolutely convinced I had chosen the correct path). When we needed an integrated website, something I already knew how to build in Perl which did not need the aforementioned library which was the reason to use Python, I kept it in Python to "keep the number of languages in active use in the company to a minimum."
In the end, I had something that technically worked, but was more akin to something an amateur programmer would produce. All my experience was worthless while I was trying to learn a new language, new libraries, and new ways of doing the things I already knew how to do.
With our new game, I almost did it again: I was going to write the server component in C++ so that it could be compiled in to the game client to provide for LAN gaming (which is another horrible excuse to learn new things while under time constraints). Lucky for me, some good old-fashioned laziness has again put me under time pressure, and the most expedient way forward is to do what I know: Perl.
Learning new things is a good idea, but it's hard to learn new things while you're also trying to get things done. If you need to get things done quickly, be very careful before deciding to use something new.