Yes, it is a common occurrence in Medicine that you feel you have "understood" something, but when you have to apply it onto a patient, you feel a little lost. But it can be overcome.

Courvoisier's law states that in the presence of a palpable gall bladder, jaundice is unlikely to be caused by gall stones.

A simpler version which may be easier to apply at the bedside:

In any patient with jaundice and a palpable gall bladder, do not consider gall stones (cholelithiasis) as a diagnosis.


Justification:
Gall stones are formed over a long period of time. This results in a shrunken, fibrotic gall bladder which does not distend easily.
Therefore, gall bladder is more often enlarged in conditions which cause obstruction of the bilary tree over a shorter period of time such as pancreatic malignancy, especially Ca of the head of pancreas.