When the recruit list of an AI side has been reduced (for example in the first scenario of World conquest when a side can only recruit from a list of 4 units), if the AI meets in its recruitment_pattern an item that matches no unit, it just stops recruiting.

This is due to a line put in comment in files:
src/ai/default/ai.cpp (line 269)
src/ai/testing/ca.cpp (line 354)

in the implementation of patch:
http://svn.gna.org/viewcvs/wesnoth?view=rev&revision=37383

(that seems unrelated to the patch itself. Commented maybe for compilation problem)

How to test: gives the AI a side with Dwarvish Steelward in World Conquest and wait until it hits the recruitment pattern "archer" when its recruit list is Dwarvish Fighter,Dwarvish Thunderer,Footpad,Dwarvish Guardsman
The thunderer is a mixed fighter while the footpad is a scout.
The default recruitment pattern for knalgans is fighter,fighter,fighter,mixed fighter,archer,scout

The error message is raised:
20101005 17:02:06 warning ai/general: At difficulty level NORMAL, trying to recruit a:archer but no unit of that type (usage=) is available. Check the recruit and [ai] recruitment_pattern keys for team ' 3' (3) against the usage key of the units in question! Removing invalid recruitment_pattern entry and continuing...