If you change your camera's resolution, remember to change it back.
During a Macworld class I was teaching, I lowered the resolution on my Canon 60D (for a particular demonstration) to "S2," which is only 1920x1280 (2.5 megapixels). After the class was over, I forgot to return the setting to Raw where I normally keep it. When I am shooting Raw, I get full resolution from the camera -- 5184x3456 (18 megapixels).
The next day, while driving to my Lynda.com assignment in Ventura, CA, I saw this dramatic scene on Interstate 5 and was lucky enough to capture it. Unfortunately, the image was at the lower resolution I had set during the Macworld class. Ack!
Well, at least I can use it on the Web...