Is there an aphorism for doing a self-defeating act?

Question

Is there a witty turn of phrase that indicates one's performing an act that, in its doing, undermines, contravenes, or obviates itself?

This question relates to a similar idea, but I have it in my mind that there is a saying that describes this state of affairs. "Cutting off one's nose to spite one's face" is not quite what I'm going for because the act of cutting off the nose doesn't negate itself. I'm thinking more of a line speaking about someone who breaks the speed limit on his way to defend himself in traffic court, or wiping off a table with a dirty sponge.

Answer 1

One may be "hoist on/by/with one's own petard" (literally, blown up by one's own bomb.) First seen in Hamlet, Act 3, Scene IV.

By the way - I received my second-ever speeding ticket on my way, not to traffic court but to traffic school, for my first. (I was young and foolish then - I'm old and foolish now.)

Answer 2

“Don’t sell your mule to buy a plow” seems to fit your description pretty well?

Although, that’s actually not one I knew until just now. I had a vague memory of a similar British one — “selling your X to pay for its Y”, or something — and googling to try to work out what it was, I found the above version, which seems to be from the southern US. I still can’t work out what the original one I was thinking of was — does the form ring a bell with anyone?

Answer 3

In Russia there is: 'Chopping a branch you're sitting on'.

Answer 4

'Shoot yourself in the foot' - perhaps?

Answer 5

How about "Eat your own head".

Answer 6

There is some overlap of this concept with the concept already discussed elsewhere on this site (http://english.stackexchange.com/questions/43237/phrase-for-focusing-on-unimportant-details) of focusing on details instead of going to the heart of the matter (e.g., “re-arranging the deck chairs on the Titanic” –or, even better, “straining out a gnat while swallowing a camel”, or – the list seems endless – “penny wise and pound foolish”…). Such behavior is self-defeating, but is a (proper) subset of the set you are asking about.

So, to cut to the chase: There is a graduation of such aphorisms/situations.

“going in circles” is the least ominous. (e.g., Pooh circling the handful of trees, tracking his own tracks, thinking he is tracking a woozel…)

“spitting into the wind” is the next step up in ominousness. (e.g., being annoying by trying to show how friendly you are)

“sawing off the branch you’re sitting on” (as noted by gasan, (and corrected by me)) is the next step up. (e.g., unions making demands that bankrupt the company that employs the workers)

being “hoist” by your own petard” (as noted by MT_Head) is the most extreme. (e.g., operation Barbarossa)

And don’t forget the classic, “If you think you’re outclassed, you are.”

And don’t forget the adage, “Argue for your limitations, and they are yours.”

And the adage, “A dog that scares many rabbits kills none.”

And we all know the story of the nurse who wakes you up to give you your sleeping pill.

And finally, if you simply want a generic expression that focuses on the stupidity/ineptness of the actor, “snatching defeat from the jaws of victory” serves nicely.