Too long to go into now and have told at least one story here before but Amazon.com had a couple fairly epic, "We switched from X to Y and Y bit us on the ass pretty hard and we went back to X," where X was Perl and Y was Java and Ruby respectively. The Ruby one was small. The Java one was millions of dollars and 18 months of roundfiled application code plus the departmental acrimony that went with the wasted time, diluted customer service, lousy code, and multiple sessions of being retrained.