One very subtle point that I've found is that alignment of the IRS on my aircraft calculates its own latitude as a reasonableness check, but not it's own longitude. The manufacturer FCOM is very hazy on alignment and error codes (none are listed any more).
However, a well known third party publication states:
Reasonableness tests
- The IRU compares entered long with the last LAST POS stored in non-volatile memory
-The IRU does not calculate its own longitude
-If the the difference you enter is greater than 1°, the ALIGN annunciation flashes, status code 4 shows and ENTER IRS POSITION displays on the CDU
-This could legitimately occur (i.e the entry is correct if the IRU was newly installed
...
The IRU also compares entered latitude with the LAST POS latitude
-The IRU calculates its own latitude
-If the difference is greater than 1°, the ALIGN annunciation flashes
This failure mode (for us humans) is very subtle.
however
I was unaware of this subtleties until this thread forced me to have a greater look at my own system knowledge and defences. The FCOM would not lead you to this understanding, and that is quite a large hole in the Swiss cheese.