diff --git a/src/markdown/parse/markdown-to-adf.ts b/src/markdown/parse/markdown-to-adf.ts
index 1d843bb..4727fc9 100644
--- a/src/markdown/parse/markdown-to-adf.ts
+++ b/src/markdown/parse/markdown-to-adf.ts
@@ -219,7 +219,7 @@ function bulletBlocks(items: readonly Block[][], reading: Reading, path: Convert
   const { nodes: blocks, refusal } = tasks.value
   if (refusal === undefined) return success(blocks)
   const place = pathTo(blocks, refusal.marker)
-  // Failures are values, so a marker the split ever copied out of sight leaves the error unplaced rather than asserting; task-list.test.ts pins that the split keeps it.
+  // Failures are values, so a marker the split ever copied out of sight leaves the error at its pre-split path rather than asserting; task-list.test.ts pins that the split keeps it.
   if (place === undefined) return { error: refusal.error, ok: false }
   const [blockIndex, ...steps] = place
   return { error: { ...refusal.error, path: [...path.slice(0, -1), listIndex + blockIndex, ...steps.slice(0, -2), ...refusal.within] }, ok: false }
diff --git a/src/markdown/parse/portable-markdown-to-adf.test.ts b/src/markdown/parse/portable-markdown-to-adf.test.ts
index 03f0ee6..48e12bf 100644
--- a/src/markdown/parse/portable-markdown-to-adf.test.ts
+++ b/src/markdown/parse/portable-markdown-to-adf.test.ts
@@ -205,6 +205,19 @@ test('names where a refused block stands after the task list splits', () => {
   assert.equal(refused('- [ ] a\n\n  <div>x</div>\n\n  tail <b>y</b>\n'), '/content/1')
 })
 
+test('holds task lists to the 500 levels the parser carries, refusing one level deeper where it stands', () => {
+  const nested = (levels: number): string => Array.from({ length: levels }, (_, level) => `${'  '.repeat(level)}- [ ] a\n`).join('')
+  assert.ok(portableMarkdownToAdf(nested(largestNesting)).ok)
+  const refused = portableMarkdownToAdf(nested(largestNesting + 1))
+  assert.deepEqual(refused.ok ? undefined : [refused.error.code, refused.error.path], [
+    'unsupported-nesting-depth',
+    ['content', 0, ...Array.from({ length: largestNesting }, () => ['content', 1]).flat(), 'content', 0],
+  ])
+  assert.ok(portableMarkdownToAdf(`${'> '.repeat(largestNesting - 1)}- [ ] a\n`).ok)
+  const quoted = portableMarkdownToAdf(`${'> '.repeat(largestNesting)}- [ ] a\n`)
+  assert.equal(quoted.ok ? undefined : quoted.error.code, 'unsupported-nesting-depth')
+})
+
 test('reads a task list beside a list of another bullet character as two lists', () => {
   const tasks = (state: string, value: string): AdfNode => bare('taskList', node('taskItem', { state }, text(value)))
   assert.deepEqual(read('- [x] a\n* b\n'), [tasks('DONE', 'a'), bare('bulletList', bare('listItem', bare('paragraph', text('b'))))])