FATAL ERROR : An unexpected condition …

FATAL ERROR : An unexpected condition occurred in file [.. wjoiner.cpp] on line

This error-message also happens when you connect  a port twice to a joiner, both to detail- and masterport …  

Have a look at the notification window in your Informatica Powercenter Designer !  

Message in the Notification Window:  

Validating data flow of mapping m_<Mapping_name>…
…transformation <..> is connected to both Master and Detail ports in Joiner <..> (Joiner port <…>)
…transformation <..> is connected to both Master and Detail ports in Joiner <..> (Joiner port <…>)
…there are data flow validation errors.    

Tested in Informatica PowerCenter 8.6.1