In November/December 2013, during the Falcon 9 Flight 7 (SES-8) processing flow, SpaceX managed to get the vehicle onto the pad and attempted to launch a few times without success. Either a scrub was called or the flight computers decided to automatically terminate the countdown prior to liftoff.
During the second launch attempt "a slower-than-expected increase in pressure" (source) was seen on the first stage engines, and an abort was called by the flight computers at T-0.
It's worth noting that this was later found to be due to contamination of the pyrophoric TEA-TEB ignition fluid, with Musk stating:
So there was some oxygen that inadvertently got in the TEA-TEB tank. Could have just been atmospheric oxygen that got in when the tank was refilled, or could have been a bit of oxygen that flowed back in from a previous engine test. That would then react with the TEA-TEB in the tank and reduce its concentration, so that the tank then contained a mix of TEA-TEB and combustion products.
In other words, it was like they expected 180 proof TEA-TEB but only got 100 proof. The fire in the engines was monitored and was less than the launch computer expected, so it shut the engines down rather than proceeding to full thrust.
On the day of launch though, Musk simply stated on Twitter:
We called manual abort. Better to be paranoid and wrong. Bringing rocket down to borescope engines ...
What exactly does "borescoping an engine" involve though? How long would this process take? What would they be able to determine with this test that they couldn't by simply looking at flight computer data?