Indiana standardized tests called into question after two days of crashes

To give a little context, Indiana's standardized test, the ISTEP, is being administered by computer for the first time. Testing was interrupted then cancelled after the web-based software provided by the vendor, CBT McGraw-Hill, crashed under the load.

I think this is noteworthy because frankly I have some questions both as a parent whose child was "impacted" (she'd call it "saved") by this and a developer. It seems like the contractor should be able to know the exact time, exact numbers and exact behavior of all their users. What are some factors that could lead to this breakdown? I want to know so I can avoid them.