I don't think you did anything wrong, to stop that from happening you have a few choices. Get that fan blowing at 100%, With my duplicator and a 40mm fan on the extruder tip I can print rather small items and cool them well enough to prevent fails. The stock fan I can't speak of, mine never worked. I also printed a blower from thingiverse to use over the stock one. The other option is to slow down your print speed with Minimal layer time, that will slow down the speed of the printer to reach that set time. Or you can enable Cool lift head in expert settings. That will move the extruder away from the print and allow it to cool.

Hope that helps!