Exactly, because if it was a purely functional language, they would have started the project, worked on it for a week, blog about it and said purely functional language for a year, then throw it away.

For example, Erlang was originally implemented in prolog.

It would not have happened if that logic was covered by tests :O

Yes and no. Erlang/OTP has a pretty extensive test suite which takes several hours to run. There is a nightly test run on a multitide of architectures. So it is some times the case that a fault is caught at night and fixed the following day.

The Erlang/OTP team runs a mantra where everything is covered by tests and they are doing a really good job at what they are doing. So I would not be surprised to see this having been caught by a test.