Software caused connection abort
This is (perhaps?) caused by recieving a RST when a SYN or SYN+ACK was expected. This is one of the few errors that accept(2) will return that are related to the connection that was being established rather than the state of the listening socket itself.
This will also happen on some platforms when connect is called a second time after connect + select fails with ECONNREFUSED (observed on Linux + Python).