diff --git a/crates/ely-pdf-engine/src/bin/ely-pdf-worker/main.rs b/crates/ely-pdf-engine/src/bin/ely-pdf-worker/main.rs
index f0f3432..4e22259 100644
--- a/crates/ely-pdf-engine/src/bin/ely-pdf-worker/main.rs
+++ b/crates/ely-pdf-engine/src/bin/ely-pdf-worker/main.rs
@@ -68,7 +68,11 @@ impl Doc {
             return Err(io::Error::other(format!("nothing to {}", if redo { "redo" } else { "undo" })));
         }
         loop {
-            mupdf.undo(&self.open, redo)?;
+            if let Err(error) = mupdf.undo(&self.open, redo) {
+                // MuPDF may have moved part way; follow it.
+                self.at = mupdf.state(&self.open)?.0;
+                return Err(error);
+            }
             let passed = match redo {
                 true => {
                     self.at += 1;
diff --git a/tasks/BUGS.md b/tasks/BUGS.md
index a9c5adc..50d45e2 100644
--- a/tasks/BUGS.md
+++ b/tasks/BUGS.md
@@ -43,7 +43,7 @@
 - [x] P-08 页面读取失败后每帧重试、每帧弹提示 — 失败的读取按页或种类记下，编辑后重读；签名读取以保存计数判过期
 - [x] P-09 识别文字的写入被拒后仍弹“成功” — on_same 回报是否执行；被拒时只有警告
 - [x] P-10 预览回复被丢弃时“应用”静默无事 — 预览回复照常交回，版本已变即提示，不显示过期预览
-- [ ] P-11 撤销或重做在 MuPDF 侧失败后 worker 的步数不再同步
+- [x] P-11 撤销或重做在 MuPDF 侧失败后 worker 的步数不再同步 — 撤销失败后重读历史标记
 - [ ] P-12 `unhook` 中途抛出时泄漏保留的对象
 - [ ] P-13 优化导出的“之前”大小取磁盘文件而非当前文档
 - [ ] P-14 文字块含控制字符时 JSON 回复不可解析