I have configured a job to generate and use a version number like this: 0.0.${BUILD_NUMBER}.${SVN_REVISION}
and place it in a variable called BUILD_VERSION.
This works nicely when only one build is executing. But if a second build of the job is started while the first is running, the content of the BUILD_VERSION variable will be empty for the duration of that build.

This is a major issue for us since it practically means we have to turn of "Execute concurrent builds if necessary" on all our CI builds.