Exporting CLASSPATH from shell can result in build failures

A few weeks ago I had exported CLASSPATH from the shell so I could manually run a jtreg test. Fast forward few weeks and I try a build in this same shell for the first time and get the following failure:

* For target buildtools_langtools_tools_classes__the.BUILD_TOOLS_LANGTOOLS_batch:
warning: [path] bad path element "/local/ws/jdk/loom/loom/build/linux-x64-slowdebug/test-support/jtreg_open_test_hotspot_jtreg_vmTestbase_nsk_jvmti_PopFrame_popframe005_TestDescription_java/classes/0/vmTestbase": no such file or directory
warning: [path] bad path element "/local/ws/jdk/loom/loom/build/linux-x64-slowdebug/test-support/jtreg_open_test_hotspot_jtreg_vmTestbase_nsk_jvmti_PopFrame_popframe005_TestDescription_java/classes/0/test/lib": no such file or directory
error: warnings found and -Werror specified
1 error
2 warnings

It was especially confusing since it was referencing a slowdebug path, and I was doing a debug build. It took a while for me to clue in what the cause was. TBH, I'm not even sure what clued me in. I think I was just lucky that the it suddenly occurred to me that I had set the CLASSPATH.