The problem arrises when the things your program depends on go away. For example, if you have a project that is written for Ruby 1.8.7, after June of this year, that version will receive no more updates. If there's some particular feature of 1.8.7 that you depend on, or some bug found, or some security vulnerability discovered, users might eventually find themselves in a spot of bother.
The web of dependencies for most projects is bewilderingly complex. The stacks that we write for are moving targets, and unless your software is moving to keep up, eventually there's a good chance that they just wont run any more.
This sort of thinking is why the IETF publishes Internet Drafts as ASCII-only text files. They've limited the dependency chain to ASCII, because they are hoping that isn't going away any time soon.