This is hydrolysis: at high temperature water reacts with the polymer and lowers its molecular weight. Sometimes you see silver streaks or bubbles, but sometimes there is no visible sign and the problem only appears in an impact test or as a failure in the customer's hands.
The allowed moisture differs for each material and must be taken from the supplier's datasheet: for polycarbonate typically around 0.02%, and for PET even lower. An ordinary hot-air dryer can't reach this level; a desiccant dryer is required.
Over-drying isn't harmless either: some materials, such as polyamide, turn yellow with long, hot drying. Temperature and time should follow the datasheet, not "the more the better".
This article is general guidance. The figures depend on each plant's conditions; for decisions, measurements on your own line are what count.