Let me start by saying that I am total noobie at QGIS, so this might be obvious to everyone else.
I spent hours yesterday trying to georeference a map. I needed to georeference it against vegetation features, so I thought it would be easiest to bring in Google Earth imagery via OpenLayers and then use the Georeferencer plugin. But it didn't work.
I finally figured out that when you bring in a layer with OpenLayers, it changes the Coordinate Reference System to something else (using UTM?) so that when you use the Georeferencer panel and click in the map, it brings in numeric entries that are absurd if interpreted as ° Lat/Lon. After you load your open layer, you need to restore the reference system.
Perhaps this is clear to all you professionals, but it took me a long time to figure this out.
I would post screengrabs, but this system won't let me.