C++ Runtime Error

[Makanyane]

Help!!!!

Have extensively modded BI 1.6

Running on -ai to test I am now getting a R6025 'pure virtual function call' C++ Runtime error at apparently random times. See pic:

Any idea if any thing I modded could have caused this (have changed text files and 2D graphics only) or would this just be from a corrupted .exe or dll's?

If so can modded text files etc. be the cause of corruption?

EDIT: have now reversed out a couple of days worth of changes, seems it could have been something in that (not entirely sure yet as above error wasn't consistent), had made changes to cultures including reassigning factions to different existing cultures and changing building.tga's for 'eastern'. Above error only came up after 80 + turns, so could it be to do with reaching certain city size / building?