If an IP is autoblocked, and you try to block it, you get the error:

xx.xx.xx.xx (talk • contribs) is already blocked (block log • unblock)

but clicking on the "block log" link doesn't show any existing blocks. The error message should note that the IP is autoblocked. If possible, also give the autoblock number.

See https://en.wikipedia.org/w/index.php?title=Wikipedia:Village_pump_(technical)&oldid=1038174132#Blocking problem