When adding a node to a cluster, I'd observed a number of AssertionErrors and some very odd errors about connections being null.
This popped up when I wasn't expecting an issue, so I'm a bit surprised. The scenario I think is:
Steps to reproduce:
1. Have two nodes running (.200 and .201 in my case here)
2. Add another node (.202 in my case here)
See the logs for what detail is available.