The most common reason an STL file may be not quite right is a mechanical connection where some piece isn’t quite as the designer expected, so the 3D model that interfaces with it no longer fits.

3Shape TRIOS intraoral scanner to export STL file digital impressions directly from the TRIOS Dental Desktop software dashboard.

Microsoft has picked up Netfabb's 3D file cloud repair service, and renamed it Microsoft Model Repair. Features faster repair times thanks to new servers.

Part of this is due to shortcomings in the STL file format. There’s a newer format available, 3MF, and Josef Prusa and Jakub Kočí would like you to start using it.