Thingiverse has some really awesome 3D models that are printable on 3D Printers!

However, not all of these great models have STL files that are correct. This is the equivalent of posting a part that is described in it's STL file like everything is in side out! Not fun! It can work on some 3D Printers - but not all - and after all one big goal of Thingiverse is sharing - "Share and Enjoy!".

The Good News: There are several freely available ways to fix these STL files!

Look at the two images at the right:

Both are prints from the UP! Both Pictures contain a broken and a fixed STL file print, the one is right off the printer - the other is after removing the support material.
Note that the broken part printed the bottom of the part in a hollow criss-cross pattern instead of being solid

1) Netfabbs cloud http://cloud.netfabb.com/ - you can upload your STL part and netfabb will repair it and let you download it and send a copy to your email address

2) Applications
Meshlab (credit nicholasclewis) http://meshlab.sourceforge.net/ - One of its many filters is "Re-orient all faces coherently". Meshlab is very nice open source, portable app.
AccuTrans3d: http://www.micromouse.ca/downloads.html - very powerful but somewhat manual process

3) Other Comment on this thing with your favorite way to repair STL files. netfabb and others have software can repair STLs in manual, semi-manual, or automatic ways.

I am not being paid or coerced to advocate for netfabb or MicroMouse/AccuTrans3d or PP3DP - the UP! company