f11bcbed9d
The original code was doing a sanity check to see if existing index was out of range. However the comparison was wrong. So if the previous ct->user (active index of texture node) was larger than then number of available texture nodes + 1 in the other material, we would never re-set the index to 0. Bug introduced onc31f74de6b. There was an early attempt of fixing this (2b2ac5d3cc) but it was just working by pure, luck. And failing in cases like the one from this bug report.