I have a multi-configuration job where "Restrict where this project can be run" is set to node1, and Configuration Matrix>Slaves is also set to node1. node1 has 1 executor.

If a build is started when node1 is online, the parent build doesn't consume an executor and the build completes successfully. If a build is started when node1 is offline, then when node1 is brought online, the parent build consumes the executor and the build is deadlocked, since the "node1" configuration has nowhere to run.

To repro:
Start with no Jenkins configuration
java -jar jenkins.war
Create node1 with 1 executor, started with JNLP. Don't connect it yet
Create multi-configuration job with both "Restrict where this project can be run" and the Configuration Matrix>Slaves set to node1
Schedule a build for the job
Run the node1 JNLP client. (It can be run on the same machine as the master)
The job never completes, as it is deadlocked