Just moving a material over the model changes material permanently.

[squashie]

This  happens when you move a material over a part but don't release.
Other parts of the model change material permanently.

Also, the UNDO button doesn't reverse this.

[edwardo]

I'v always had problems with 'undo' in most cases, or rather, i'v never really got my head around its nuances.

[squashie]

I'v always had problems with 'undo' in most cases, or rather, i'v never really got my head around its nuances.

I don't have problems for most part. This just happens on larger models. That's what it seems like anyway.