Is there an expression to describe "waiting for someone to make a mistake so as to criticize them harshly"?

Question

I am looking an expression that could mean "eager for someone to make a mistake so as to criticize them?". This is to be used in the instance where a famous personality has a lot of critics, probably jealous of him. They read too much into what he writes and tries to criticize him. They are eager for his new writing with the sole purpose of criticizing his writing and him.

Example:

You should be very careful when you go on twitter. There are people _________________________________ (who expect you to make a mistake so as to criticize you).

I was wondering if there is something like "thirsty for you blood", not sure.

Answer 1

ready to pounce

In the OP's example:

You should be very careful when you go on twitter. There are people ready to pounce on anything you say that can be misunderstood or criticized.

The image is of a cat ready to pounce on a bird or a mouse -- or a lion or tiger ready to pounce on a deer. See Images for ready to pounce.

pounce. Oxford Living Dictionaries. See especially the third definition below.

1 -- (of an animal or bird of prey) spring or swoop suddenly so as to catch prey:

‘as he watched, a mink pounced on the vole’

1.1 -- (of a person) spring forward suddenly so as to attack or seize someone or something:

‘the gang pounced on him and knocked him to the ground’

1.2 -- Notice and take swift advantage of a mistake or sign of weakness:

‘the paper pounced on her admission that she is still a member of CND’

Answer 2

'drop the ball'

In the template provided:

You should be very careful when you go on twitter. There are people waiting for you to drop the ball_ (who expect you to make a mistake so as to criticize you).