When in.Hg is used for altimeter setting, unusual barometric pressures such as:
28.XX in.Hg (i.e., an unusually low pressure); or,

30.XX in.Hg (i.e., an unusually high pressure),

may go undetected when listening to the ATIS or ATC transmissions, resulting in  a more usual 29.XX altimeter setting being set.
A 1.00 in.Hg discrepancy in the altimeter setting results in a 1000-ft error in  the intended (actual) altitude, as illustrated by Figure 1.
Note
Figure 1, Figure 2 and Figure 3 assume :
a 2000 ft airfield elevation; and,

a 4000 ft indicated altitude.

In Figure 1, the actual QNH is an unusually low 28.XX in.Hg but the altimeter setting was mistakenly set to a more usual 29.XX in.Hg, resulting in the actual altitude / height being 1000 ft lower than indicated:
 
 
 
 
 
 
 
Figure 1
Effect of a 1.00 in.Hg Too-High Altimeter Setting
Actual height1000 ft AFLAltimeter setting .XX in.HgActual altitude3000 ftAltimeter error 1000 ftActual height1000 ft AFLAltimeter setting .XX in.HgActual altitude3000 ftAltimeter error 1000 ft Actual height1360 ft AFLAltimeter setting 29.in.Hg ( 1012hPa)Actual altitude3360 ftAltimeter error640 ftActual height1360 ft AFLAltimeter setting 29.in.Hg ( 1012hPa)Actual altitude3360 ftAltimeter error640 ft

In Figure 2, the actual QNH is an unusually high 30.XX in.Hg but the altimeter setting was mistakenly set to a more usual 29.XX in.Hg, resulting in the actual altitude / height being 1000 ft higher than indicated.
 
 
 
 
 
 
 
 
Figure 2
Effect of a 1.00 in.Hg Too-Low Altimeter Setting
Similarly, a 10 hPa error in the altimeter setting would result in a 300 ft error in  the actual altitude (i.e., with a 10 hPa too high altimeter setting, flying at a 4000 ft indicated altitude would result flying at a 3700 ft actual altitude).
Confusion between altimeter setting units (i.e. hPa versus in.Hg) leads to similar errors in the actual altitude and actual height above airfield elevation.

In Figure 3, an actual QNH of 991 hPa was mistakenly set on the altimeter as 29.91 in.Hg (equivalent to 1012 hPa), resulting in the actual altitude / height being 640 ft lower than indicated.
 
 
 
 
 
 
 
 
Figure 3
Effect of an Altimeter Setting in in.Hg Instead of hPa
Setting the Altimeter Reference

In order to eliminate or reduce the risk associated with the use of different altimeter-setting units or with the use of unusual (low or high) altimeter-setting values,  the following rules should be used by controllers (when recording the ATIS message or when transmitting the altimeter-setting) and by pilots (when reading back  the altimeter-setting):
　
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