During an LPC/OPC, I completely messed up my cold wx correction calculations. No one, not my sim partner (also a trainer) or the company TRE noticed. Just goes to show, how important this really is in over 99.9% of approaches.
In practice we fly through an air mass which is at temperature x, yet our calculations are based on y, the temp on the ground. There is zero room for a temperature inversion in the standard calculation. Anyone flying into Scandi land in the middle of winter knows the temp on the ground is often colder than what the OAT was at platform (extra cooling effect of ground snow/ice).