What Have You Ignored?

For a long time, I never really used the International Genealogical Index (IGI) on the FamilySearch site very much. Most of my ancestors were German, Irish, or Early American and I just never found anything that I didn't already know and filtering through all the erroneous entries diverted me from more productive pursuits, in my opinion.

However, when I started working on my wife's English immigrants I've had to change my tune. The IGI includes significant extractions from English parish records and they have helped me refine my searches. Keep in mind that the IGI is a secondary source and that in the cases where I was working they usually didn't extract deaths, mainly births and marriages. And one should not assume they have every parish. And they don't extract father's occupation and specific residence which may also be on the original record. The IGI is not an end in and of itself and information it contains should be verified with actual sources.

Is there some source you've been in the habit of ignoring that perhaps you should start checking?