In this segment from early June Samantha Bee talks about cheap drug test kits used in the field by police that are generate a lot of false positives.
In theory the kits are supposed to be be used only as a preliminary screening test to avoid spending a lot of money on ordinary substances that only look like they might be illegal drugs. In practice, it's not uncommon for people to be locked up on the basis of a preliminary test and then pretty much coerced into a false guilty plea because getting reliable test results can take so long that the innocent arrestees can actually spend a lot less time in jail by pleading guilty than by waiting for proof of their innocence to come in. Also, too many courts accept field test results as if they were accurate.
But if you've got money, you can hire a lawyer, get bonded out, get a test done at your own expense, and get the case thrown out. If you don't, you can get a public defender who doesn't have time to listen to you and a judge who just assumes you're guilty because hey, you're poor, especially if you're also not white, and if you try to plead not guilty he tells you he's going to sentence you to 20 years.
Think that's an exaggeration? Watch what happens below to two innocent people, only one of whom has money. The poor guy's test results don't come back until nearly a year after gets out of prison, and they say the trace substance found in his car wasn't a controlled substance after all. And even then, it takes another five years for him to be exonerated.