[jwilk]

> sed -n 's/old/new/wfile.tmp' file

This saves only lines which underwent substitution, which was probably not what you wanted.

[1vuio0pswjnm7]

Missing semicolon.

  sed -n 's/old/new/;w file.tmp' file

Unlike BSD and GNU sed, it appears that Plan9 sed will append to instead of overwite file.tmp

It also requires a space after the w command.