Most people don't value 99.999 vs 99.9999 reliability as worth millions of dollars per system per year. Space shuttles may disagree, but not billing platforms.
For something like the elevons, control was accomplished by connecting 3 actuators to 3 of the channels, with voting being accomplished by physical force - if the 3 computers disagreed, the 2 would overpower the one. (Things like thrusters used electronic voting, close to the thruster itself.)
This seems closer to modern architectures with multiple computers, than the mainframe idea of redundant hardware.
"At 12:12 GMT 13 May 2008, a space shuttle was loading its hypergolic fuel for mission STS-124 when a 3-1 disagreement occurred among its General Purpose Computers (GPC 4 disagreed with the other GPCs). Three seconds later, the split became 2-1-1 (GPC 2 now disagreed with GPC 4 and the other two GPCs). This required that the launch countdown be stopped.
During the subsequent troubleshooting, the remaining two GPCs disagreed (1-1-1-1 split). This was a complete system disagreement. However, none of the GPCs were faulty. The fault was in the FA 2 Multiplexer Demultiplexer. This fault was a crack in a diode. This crack was perpendicular to the normal current flow and completely through the current path. As a crack opened up, it changed the diode into another type of component ... a capacitor.
Because some of the bits in the signal are smaller than they should have been, some of the GPC receivers could not see these bits. The ability to see these bits depends on the sensitivity of the receiver, which is a function of manufacturing variances, temperature, and its power supply voltage.
From the symptoms, it is apparent that the receiver in GPC 4 was the least sensitive and saw the errors before the other three GPC. This caused GPC 4 to disagree with the other three. Then, as the crack in the diode widened, the bits became shorter to the point where GPC 2 could no longer see these bits; which caused it to disagree with the other GPC. At this point, the set of messages that was received correctly by GPC 4 was different from the set of messages that was correctly received by GPC 2 which was different again from the set of messages that was correctly received by GPC 1 and GPC 3. This process continued until GPC 1 and GPC 3 also disagreed with all the other GPC."
[1] Adapted from https://c3.nasa.gov/dashlink/projects/79/wiki/test_stories_s...
There's a bit of debate as to what would have happened if it ever took over.
Years ago, I had a customer that had a Tandem which was quite exciting to me as I had yet to encounter one up to that point of my career.
Imagine my letdown when I eventually discovered that the Tandem was used for FTP. ¯\_(ツ)_/¯
And if it's a software problem, then again the redundancy isn't helping you.
Basically you haven't succeeded in getting 5 independent tries in the problem -- the failures are highly correlated.
It's the same reasoning that led to the financial crisis -- you have a CDO or an MBS that has a bunch of different obligations so you think you've diversified away risk, but in actuality there is one big factor that causes everything to fail at once.
Same brand, same batch, operating time, running temperature, even same vibration does trick. :(
For highly critical systems that is an issue but it is being adressed. Take as example the space shuttle or the Airbus (3/2) primary/secondary flight computer [1] where the backup is being build by different companies with different processors ...
[1] https://ifs.host.cs.st-andrews.ac.uk/Resources/CaseStudies/A...