Something that looks like a good thing at first, but has unforeseen bad consequences.

For instance, while irrigation schemes provide people with water for agriculture, they can increase waterborne diseases that have devastating health effects, such as schistosomiasis. (From wikipedia)

_{Major edit in body to provide a better example as requested in the comments. -T.R.}

Something that looks like a good thing at first, but has unforeseen bad consequences.

For instance, while irrigation schemes provide people with water for agriculture, they can increase waterborne diseases that have devastating health effects, such as schistosomiasis. (From wikipedia)

_{Major edit in body to provide a better example as requested in the comments. Saeid, please let us know if you disagree and this conflicts with your intent. -T.R.}

John Forbes Nash, Jr. was an American mathematician who was killed in a traffic automobile accident, because the car service he had ordered didn't show up and he hired a taxicab and crashed on the way. This is the opposite of idiom “blessing in disguise” that a misfortune unexpectedly turns into good fortune; so what idiom or saying describes the opposite situation?

Something that looks like a good thing at first, but has unforeseen bad consequences.

For instance, while irrigation schemes provide people with water for agriculture, they can increase waterborne diseases that have devastating health effects, such as schistosomiasis. (From wikipedia)

_{Major edit in body to provide a better example as requested in the comments. -T.R.}