But what is obviously bad or flawed evidence? What is the standard of reliability? What should not be admitted, and what should be admitted and subject to vigorous challenge?
Suppose one considers bullet markings (left by contact between the bullet and high points in the barrel during the bullet's travel down the barrel).
The prosecutor will claim that the markings are from the defendant's gun. There will be a control firing by the police with that gun for a comparison bullet. How closely should the two sets of markings match to be considered showing that the fatal bullet was fired from the same weapon as the control?
How many other guns of the same caliber, possibly from the same manufacturer, would also produce similar or even identical markings?
Now one could argue that all of the bullet evidence should be admitted, but also the defense must know the conditions of obtaining the control and how the comparison was done. The defense should have the right to conduct its own control firing, IMO. Furthermore, the prosecution needs to answer the question of the relative uniqueness of the markings and the defense should be able to counter with its own evidence.
What about evidence in which the chain of custody has been broken? Should that be eligible for introduction into a court's evidence? What about evidence that has been collected in a manner that compromised it? IMO, NO, for both questions.
But what about a test such as DNA profiling, or mass spectrometry or other test to identify the chemical constituents of some drug or presumed poison? Such tests are far more complex and subject to error, contamination, and even manipulation. So discovery of the methods, equipment, and material are essential. Furthermore, the validity of the test must be established by the prosecution.
If the test has no history of reliability that can establish its validity, perhaps since it was just developed, it should not be admitted, IMO. Any appearance of a result against the defendant would be prejudicial, unless perhaps the jury was composed of individuals both skeptical and capable of understanding the complexity of the scientific issues of the test. So that is one example of the issue of reliability and introduction of evidence. Generally in the US, LCN DNA testing is not even attempted for forensics, AFAIK, because of concerns about reliability and validation. The FBI has a publication including requirements for laboratory validation:
http://www.fbi.gov/about-us/lab/biometric-analysis/codis/qas_testlabs
Here is an excerpt from the validation section:
Standard 8.1
The laboratory shall use validated methodologies for DNA analyses. There are two types of validations: developmental and internal.
Standard 8.2
Developmental validation shall precede the use of a novel methodology for forensic DNA analysis.
8.2.1 Developmental validation studies shall include, where applicable, characterization of the genetic marker, species specificity, sensitivity studies, stability studies, reproducibility, case-type samples, population studies, mixture studies, precision and accuracy studies, and PCR-based studies. PCR-based studies include reaction conditions, assessment of differential and preferential amplification, effects of multiplexing, assessment of appropriate controls, and product detection studies. All validation studies shall be documented.
8.2.2 Peer-reviewed publication of the underlying scientific principle(s) of a technology shall be required.
Standard 8.3 Except as provided in Standard 8.3.1.1, internal validation of all manual and robotic methods shall be conducted by each laboratory and reviewed and approved by the laboratory’s technical leader prior to using a procedure for forensic applications.
So if the lab doesn't meet some minimum requirements, there isn't validation, and the lab doesn't have QA or contamination control, don't bother to admit its alleged evidence. And if by error it had been admitted, toss the case on appeal.