Prosecutors often obtain drug convictions through means such as eyewitness testimonies and police officer testimonies. However, witness testimonies can frequently be problematic. For one thing, the witnesses might have incentives to lie. Perhaps they believe that if they testify a certain way, they will get years knocked off a sentence.
The reality is that the drug case against you might not be as cut-and-dried as police and prosecutors may have made it seem.
They have incentives to lie
Getting a lesser sentence, no conviction at all or a similar type of reward can be a reason for witnesses to testify, either honestly or dishonestly, to certain events. During earlier questioning at the police station, police officers may have used tactics such as, "If you confess before XYZ confesses, you will get a lot less time in prison."
They are police officers
Some police officers are upstanding people, but many officers color outside of the lines and much worse. Police officers have been guilty of planting evidence and of lying on the witness stand. Perhaps they do so to help a police friend out or to seal a conviction they think the defendant deserves. Similarly, they might enter premises without a warrant, find "evidence" and say it was in clear sight from the window to justify entering without a warrant. Some police officers bend the truth throughout the whole process of a state or federal criminal case, and lying on the witness stand is one part of that.
They are human
There is also the fact that there are everyday, ordinary, human problems with eyewitnesses. Someone might have witnessed a drug delivery and identified you 100 percent as the person making the delivery despite it being, say, a cross-racial identification made in dim light and perhaps even without the witness wearing his or her everyday eyeglasses. Many witnesses who truly believe that they made the right identification have been proven wrong later.