"CL happily compiles it, and if bar is never defined, a runtime error will occur. ... You try to import other-package and, BAM!, another error - conflict, other-package:bar conflicts with read-in-package:bar. Then you go learn about uninterning.
fwiw, you get a warning at compile time regarding the undefined symbol. And in the AllegroCL IDE, for example, uninterning is just a matter of hitting [Return] when you get the error message (dialog). I believe uninterning the conflicting symbol is a common restart in other implementations as well.