An error is printed if an input source map is used.

[JoolsCaesar]

Bug report or feature request?
Bug report

Whenever you use an input source map, so you can map back to an original set of files an "INFO" message is output on stderr. It seems strange that this is output in an otherwise quiet run, let alone on stderr.
It looks to be coming from this line:
print_error("INFO: Using input source map: " + settings.content);

Uglify version (uglifyjs -V)
uglify-es 3.3.10

JavaScript input
Anything

The uglifyjs CLI command executed or minify() options used.
uglifyjs input.js -o output.js --source-map "content=input.js.map"

JavaScript output or error produced.
INFO: Using input source map: input.js.map