Many people assume that testing for drugs is a relatively straightforward process. We envision a machine where you put something in, and it tells you yes or now. The truth is testing is not as simple as that. A number of processes are involved, which means mistakes will be made.
It is probably impossible to determine how often mistakes are made. Even if it's a small percentage of cases though, if it's your case the mistake can mean the difference between living with criminal record, or getting the charges dismissed.
When the lab does do everything right, sometimes they determine that what was submitted was not a controlled substance. Every few weeks I see a case where someone pled guilty, only to later find out that whatever they had wasn't a controlled substance at all. Examples of that include Ex Parte Davenport-Fritchse, No. AP-77,013 (Tex. Crim. App. - 4/17/13) and Ex Parte Gainus, WR-79,346-01 (Tex. Crim. App. - 5/8/13) In the first case the defendant was sentenced to 60 days, and in the second case three years.
I still don't understand how that happens. In Waco and McLennan County, the District Attorney will not file a felony case without having a lab report, which would hopefully solve that problem. I would imagine a number of other counties have the policy. It only makes sense to make sure you are actually prosecuting a crime before moving forward. Otherwise you end up with cases like those above.
I also don't understand why a defendant would agree to do that. Were they duped, and thought they had actually had something? I realize drug dealers are probably not the most trustworthy people out there, but do they sell fake stuff that often? You would think it would be pretty easy to figure out.
The lesson from all of this is to not take anything for granted. If the lab report is back, look behind it and make sure everything was done properly. If it's not, don't go forward until you have it.