What did you do?

See https://godoc.org/golang.org/x/tools/present. The line
"at the beginning of a line or else be preceded by a space or punctuation; similarly a closing marker must be at the end of the lineo"
appears just before the code box showing marker examples. It appears to be a duplicate of a line in the middle of the previous paragraph.

What did you expect to see?

Nothing

What did you see instead?

A duplicate line without a proper full stop.