I Cite So I Know What Was in My Sight

One of the reasons genealogists cite their sources is so that they know what version of something they used in case another rendering of the same original information comes along.

A good example is the National Archives microfilm publication M1916--which includes microfilmed images of the front of cards that were completed in application for tombstones for deceased veterans. The cards were microfilmed in black and white. Images of the entire card (front and back) are available in color digital format on Ancestry.com. If my citation indicated that I "just" used the microfilm, then I know that perhaps the color images will tell me something I don't know. If my citation indicates that I used the color image at Ancestry.com, then I probably don't need to see the microfilm.

If my citation just says I "looked at the card" and doesn't indicate which version, then I don't know if I need to look at the other version or not.

Here's a post that discusses this database in a little more detail.