When it comes to fall out from one module within my campaigns its got to be X1 Isle of Dread by
Races such as the as the racoon-like phanaton and the cat-like rakasta give X1 part of its "Lost World/Skull Island laboratory feel'
The Elder Things from H.P. Lovecraft's 'At The Mountains of Madness' made interplanar war on a ton of alien races. But we as humanity don't know the scale of such a war which technically could be on going. The reason that the isle of dread is in the plane of water is because the Elder Things sent it there! The resulting disaster took out the Kopru. Now I've covered this before on this blog in 'Another Mini OSR Campaign Idea Using - X1 Isle of Dread By David Cook & Tom Moldvay & Some Free Old School Appendix N Downloads'