Issuing a HEAD request results in the connection not being put back into the urllib3 pool. This is tied to this issue in urllib3: urllib3/urllib3#673. The problem is resolved in urllib3 >= 1.11. However, botocore vendors requests 2.7.0, which vendors urllib3 1.10.4. Updating the vendored requests library to >= 2.8.0 would resolve this issue.

Relevant issue in urllib3: urllib3/urllib3#1234