Poisoning the well may be thought of as an Ad Hominem fallacy in advance. If poison is dumped into your well, you don't drink from it. Similarly if someone poisons your mind about X, by telling you something unfavorable about B, you may be inclined to reject anything that X says to you.
This is a rhetorical device where adverse information about a target is presented to the audience. The objective is to ridicule or discredit everything the target may say.
Someone tells you something unfavorable about X
Therefore X's claim will be false
You reject what X says, since your mind has been poisoned about X
X tells Y: The new boss got sacked from his last job
New boss to Y: Here's the new marketing plan, Implementation will result in a 5% increase in market share
Y to X: The new boss told me about his marketing plan. How can it work when he obviously got sacked for incompetence from his last job
X has poisoned Y's mind about the new boss
Y now believes the marketing plan of the new boss will not work.
Y is now a victim of the poisoning of the well by X
Variations of this happen in the workplace all the time. For all Y knows, the new boss may not have been sacked at all, or left because of a difference of opinion. And even if the new boss got sacked on account of the work done, it is no reason to discredit the new marketing plan based on what happened in another job. A critical thinker would evaluate the marketing plan in the right context without being influenced by hearsay from X
Examples from the 2014 Election campaign in India
"On the birth anniversary of BR Ambedkar, BSP chief Mayawati on Monday attacked BJP leaders saying when polls come they see Dalits as Hindus but later they do not even like to sit with them, and the party was not happy when the architect of the Constitution got Bharat Ratna" - TOI
Priyanka Gandhi drinks too much alcohol and has a bad name, says Subramanian Swamy
Commenting on reports of Congress President Sonia Gandhi's daughter Priyanka Gandhi's willingness to contest elections from Varanasi, Bharatiya Janata Party (BJP) leader Subramanian Swamy on Monday said that Gandhi would have been thrashed if she had done so as she drinks too much alcohol and has a bad name.