Bad batches of generic drugs?

Hello all, I was hoping to hear some opinions about whether there are variations between batches of different prescription drugs. The pharmaceutical companies maintain that their generics are identical to the brand name and that the quality of the pills is absolutely consistent.
Yet, many patients are convinced they experienced negative side effects due to bad batches of their regular medication and that the symptoms resolved when they started a new lot.

The highest frequency of complaints seems to be about anti-depressants- particularly generic wellbutrin (bupropion). Some of you will believe most or all of those individuals are blaming the medication for some other variable. However, it was shown early on that the time-release generics were releasing their active ingredients far too quickly- so subtle compositional differences do have an affect.
All comments welcome.