Brewer's: Viper and File
The biter bit. AEsop says a viper found a file, and tried to bite it, under the supposition that it was good food; but the file said that its province was to bite others, and not to be bitten. (See Serpent.) The viper of real life does not bite or masticate its food, but swallows it whole.
I fawned and smiled to plunder and betray, Myself betrayed and plundered all the while; So gnawed the viper the corroding file.
“Thus he realised the moral of the fable: the viper sought to bite the file, but broke his own teeth.” —The Times.