libcurl built on top of NSS (Network Security Services) incorrectly re-used client certificates if a certificate from file was used for one TLS connection but no certificate set for a subsequent TLS connection.
While the symptoms are similar to CVE-2016-5420 (Re-using connection with wrong client cert), this vulnerability was caused by an implementation detail of the NSS backend in libcurl, which is orthogonal to the cause of CVE-2016-5420.