When Agents (and theoretically NPCs) encounter unnatural beings, they usually have to make a SAN test and risk losing some SAN. Most SAN losses in Delta Green follow a rule of thumb set out in the Agent's Handbook's Sanity chapter.
If the cost of failing a SAN test is 1D6 or less, the cost of success is usually zero. If the cost of failure is 1D8 or 1D10, the cost of success is usually 1. Some unnatural events and encounters are even more catastrophic (AH, 66).
You can map out the progression of SAN loss like so: 0, 0/1, 0/1D4, 0/1D6, 1/1D8, 1/1D10, 1D4/1D12, 1D6/1D20, and 1D10/1D100. We don't find 1D10/1D100 until we open the Handler's guide, and 1D4/1D12 is purely a theoretical SAN loss that fits very nicely between 1/1D10 and 1D6/1D20 but I've never seen it in any published DG material.
EDIT: as of Fall 2021, we actually now have an official 1D4/1D12 SAN loss in ARCHINT.
Some listed SAN losses deviate from this progression. Most of these anomalies are unnatural, but first I'll cover the mundane anomalies found in the Agent's Handbook.