Steps to reproduce:

1) Create a new maven project and make sure it is configured to run in parallel (use my provided config.xml for reference).
2) Execute 4 builds at the same time and wait for them to complete
3) Run another build after the 4th one complete, notice that the next build number will be skipped!

I enabled the hudson.maven (ALL) logger and extracted the following details in my steps to reproduce. I've attached the output to this ticket. Notice that build #36 is skipped.

I also checked the nextBuildNumber file and noticed that it was set to 37!