This bug is the cause of some of the problems in Flopsy's recent bug report. (http://www.lemmingsforums.net/index.php?topic=3026.0)
Assume a level has p preplaced lemmings and n hatches (indexed by 0, 1, ..., n-1). Then the first lemming should spawn in hatch number 0, but instead is spawns in hatch number p mod n.
...and fixed for the next NL update.