When you get that feeling that what you're doing is "like watching cable, only with fewer hair replacement infomercials"[1]. It's noticing the particular kind of boredom you get when you're slipping into a not very interested consumerist attitude.
I have generally treated this as a signal to stop bothering with whatever it is. Could do with strategies for finding more interesting new things to do.
(I find myself uninterested in trying to get more interested in the not-sufficiently-interesting thing, and in fact dislike the idea of doing so. I'm not entirely clear why. Possibly it's a slight feeling of resentment at the whatever-it-is for wasting the seconds of my life. There is an inexhaustible supply of things that would just love one's precious attention, and I feel a need to be harsh in culling them.)
[1] Dunn, Sarah. The Official Slacker Handbook. Abacus, 1994. ISBN 0-349-10591-X
In Anna’s recent post, she talked about training your mind to notice when it wasn’t curious about something and scream “Error! Look for a different way to do this” in such cases. Johnicholas and TheOtherDave's list of what stupidity feels like also looks useful for this purpose. I'm creating this post to make a more comprehensive list of feelings which indicate that people should reanalyze different possible paths to make sure that the one which they're taking is the most effective one to their objective.
Please suggest additions to the list in your comments -- I'll move them up here (along with links to further explanation, if given.) Keep in mind that your description of the feeling should be as illustrative as possible. For example, "feeling stupid" is unhelpful, while "you feel like you've taken a wrong turn into a never-ending tunnel" is better. Of course, metaphors which are immediately understood by some people may not be so easily understood by others, so try to give a more detailed description of the feeling if other people express that you're probably saying more than they're hearing.
List: "Error! Look for a different way to do this" if you feel like: