diff --git a/README.md b/README.md
index 623671e..80cd310 100644
--- a/README.md
+++ b/README.md
@@ -62,6 +62,36 @@ Every one of those is a fix somebody made once, in one repo, that the other
 will either never get or get by being written a third time. That is the
 cost this repo removes, and it is already being paid.
 
+## It found a bug in all three before shipping a line
+
+`private` was lifted from ai-app because that copy was the better one. Two
+sessions then read it as a module rather than as scattered helpers, and
+found that `OpenOptions::mode` applies **only when a file is created** — so
+opening one that already exists keeps whatever mode it had.
+
+Both projects rewrite existing files holding private material. `certs.rs`
+reissues the TLS leaf on every start, so a `leaf-key.pem` that had ever
+existed world-readable would have stayed that way for the rest of its life,
+with every start looking like it was setting the mode. The config's temp
+file is the other: normally fresh, but one left by a crashed save is
+reopened with its old mode and then renamed over the file holding the
+enrolled token hashes.
+
+It was latent in both — the modes on both machines were checked, and were
+correct — but it was present three times: dev-updater, ai-app, and this
+crate's first commit.
+
+The reason nobody caught it is the argument for this repo, and it is not
+about duplication. The correct reasoning was already written down, in a
+comment, three functions above the code that needed it: `create_dir` sets
+the mode a second time and explains exactly why. It stayed invisible for as
+long as nobody had cause to read those functions as a set. Two readers
+looking at one module found it in under an hour.
+
+Fixed here (`67ac924`), in ai-app (`d0b6b66`), and in dev-updater
+(`bdaade4`), which also adopted the `private` module wholesale — the first
+time the sharing paid rather than merely being argued for.
+
 ## What is here
 
 `server/` — the `wg-link` crate. Three modules, each extracted only after