This is why dimensioned PDF drawings are so extensively used in procurement. The top photo is from a supplier from MFG.com; the bottom one is the STEP that I originally uploaded to MFG.
This supplier has obviously downloaded my STEP and performed some translation or conversion on it, and in the process has deleted a few faces (you can see the difference in the foreground of the part, and in the areas he's highlighted in red). This is clearly an inexperienced supplier, and one that I would ultimately have a *really* hard time choosing. I'm not sure what he did or how he did it, but the fact that he made this mistake is an indicator that we'd have issues down the road.
(To his credit: the photo came from a message he wrote me saying that he "noticed some missing surfaces on the part file," and asking me to fix them. So he knew that there was a problem, but didn't understand what it was and wasn't able to troubleshoot it himself.)
In traditional manufacturing, 3D part files are created and edited in a program like Inventor or SolidWorks. The parts are then brought into a separate environment in the same application and drawn and annotated in multiple 2D views on a "paperspace." The resulting drawing file (.IDW for Inventor) is a dynamic representation of the original part; if you modify the part file, the drawing will update automatically.
You *never* submit drawing files directly to a manufacturer. Instead, you export PDFs of the dimensioned drawings, and *optionally* include STEP files (which are essentially cross-platform 3D files) as a courtesy. The STEPs can be used to help the manufacturer set up their CNC machines, but they're for reference only; the PDFs (with all their dimensions and annotations) are what you're buying.
"Organic" shapes - like those that 3D printing is so well equipped to make - don't fit into this process well. Complex surfaces are *really* difficult to define clearly and completely in two dimensions, and so most 3D printed parts are built from solid files. In this case I submitted a STEP, which manufacturers will convert to an STL and then run through a slicer and feed into their machines.
The problem is that STEP files aren't immutable, and the supplier in this case has apparently deleted a feature from the part. In this case the result was obvious, but there are a lot of features that he could modify or delete that would be a lot more difficult for him to detect, and my QC job would be accordingly tricky.
This process should be better. The PDF workflow is inconvenient, but at least it's an effective barrier to issues like this one.
Also, we need more, and more *good*, DMLS suppliers.