Currently when looking for which exp value to use from the soft_level_reqs list in variables.gd, the method checks first that there is a valid index:

if exparray.size()-1 < abs(growth_factor * variables.class_cap_per_growth + variables.class_cap_basic):

It should be:

if if exparray.size()-1 < currentclassnumber

The current version causes the game to use the last number in the list if the growth variables are too large regardless of the actual number of classes

Of course, if one is fiddling with the variables one can always just make the list long enough to be bigger than the max number of soft_req classes, For anyone who wants fo use the fix above, in version 0.6.0b the method is found on line 90 in ch_leveling.gd and the relevant line is 99.