https://phys.org/news/2019-01-d-printed-guns-dangerous-users.html

As someone who uses 3-D printing in his work and researches quality assurance technologies, I've had the opportunity to see numerous printing defects and analyze what causes them. The problem is not with the concept of 3-D printing, but with the exact process followed to create a specific item. Consumer 3-D printers don't always create high-quality items, and regular people aren't likely to engage in rigorous quality assurance testing before using a 3-D-printed firearm.

Many consumer 3-D printers experience a variety of glitches, causing defects in the items they make. At times, an object detaches from the platform it's on while being made, ending up lopsided, broken or otherwise damaged. Flaws can be much harder to detect when the flow of filament – the melted plastic material the item is being made from – is too hot or cold or too fast or slow, or stops when it shouldn't. Even with all of the settings right, sometimes 3-D-printed objects still have defects.