Tetanus doesn't grant immunity if you actually get it and survive. They are soil/intestinal bacteria normally and they don't grow within you to a high enough number that your immune system can get a good look at them, their toxin is just potent enough that even at low concentrations it kills you.

There are also protist pathogens which express vast quantities of a particular coat protein on their surface such that when you form an adaptive immune response agains them it is almost certainly against that protein - and something like one in 10^9 cell divisions their DNA rearranges such that they start expressing a different coat protein and evade the last immune response that their host managed to raise, resetting back to no immunity.