I remember watching a friend play IWD at his place and watching his gnome T/I run freely back and forth thru these spike stones that his druid had laid down. Am I just imagining things, or did this actually occur? Was it an item or a spell that allowed the fella to do this?

maybe he made his save, or had some spell immunity (magic resistance, or globe of invulnerability)

I know that my hafling makes a saving throw quite often agains spike growth. I use him sometimes to draw in more enemies.