diff --git a/src/world/index.ts b/src/world/index.ts
index 9e0acab..fc38f95 100644
--- a/src/world/index.ts
+++ b/src/world/index.ts
@@ -51,12 +51,31 @@ const endings: World['endings'] = {
   wrong: { whenFlags: {}, narration: '' },
   bad: { whenFlags: {}, narration: '' },
 }
+const seenEndings = new Set<string>()
 for (const [path, raw] of Object.entries(endingFiles)) {
   const { id, ending } = parseEnding(raw, path)
   endings[id] = ending
+  seenEndings.add(id)
+}
+const requiredEndings = ['true', 'wrong', 'bad'] as const
+for (const id of requiredEndings) {
+  if (!seenEndings.has(id)) {
+    throw new Error(`endings/${id}.md is missing — every ending id must have a markdown file.`)
+  }
 }
 
 // Cross-reference validation.
+// Build set of all known flag names from encounter setFlags and ending whenFlags.
+const knownFlags = new Set<string>()
+for (const enc of Object.values(encounters)) {
+  if (enc.onResolved?.setFlags) {
+    for (const flagName of Object.keys(enc.onResolved.setFlags)) knownFlags.add(flagName)
+  }
+}
+for (const ending of Object.values(endings)) {
+  for (const flagName of Object.keys(ending.whenFlags)) knownFlags.add(flagName)
+}
+
 for (const room of Object.values(rooms)) {
   for (const [dir, dest] of Object.entries(room.exits)) {
     if (!rooms[dest!]) {
@@ -71,6 +90,20 @@ for (const room of Object.values(rooms)) {
   if (room.encounter && !encounters[room.encounter]) {
     throw new Error(`rooms/${room.id}.md: encounter "${room.encounter}" is not defined`)
   }
+  if (room.lockedExits) {
+    for (const [dir, lock] of Object.entries(room.lockedExits)) {
+      const isItem = items[lock.requires] !== undefined
+      const isFlag = knownFlags.has(lock.requires)
+      if (!isItem && !isFlag) {
+        const knownItemList = Object.keys(items).join(', ') || '(none)'
+        const knownFlagList = [...knownFlags].join(', ') || '(none)'
+        throw new Error(
+          `rooms/${room.id}.md: exit${dir.toUpperCase()}Requires "${lock.requires}" matches no known item or flag. ` +
+          `Known items: ${knownItemList}. Known flags: ${knownFlagList}.`,
+        )
+      }
+    }
+  }
 }
 
 // Validate encounter narration registry: every encounter in TS has a markdown doc.
diff --git a/src/world/loader.test.ts b/src/world/loader.test.ts
index 0d8d04f..90e39f2 100644
--- a/src/world/loader.test.ts
+++ b/src/world/loader.test.ts
@@ -310,3 +310,55 @@ describe('narration registry', () => {
     expect(() => narration('ghost', 'whatever')).toThrow(/unknown encounter id "ghost"/i)
   })
 })
+
+describe('parseRoom invalid headers', () => {
+  it('throws a clear error when a header has spaces', () => {
+    const md = `---
+id: r
+title: "[ R ]"
+exitN: null
+exitS: null
+exitE: null
+exitW: null
+exitU: null
+exitD: null
+items: []
+---
+
+## first visit
+This is the first visit.
+## revisit
+.
+## examined
+.
+`
+    expect(() => parseRoom(md, 'rooms/r.md')).toThrow(
+      /invalid section header "## first visit".*letters, digits, hyphens/,
+    )
+  })
+
+  it('throws a clear error when a header has dots', () => {
+    const md = `---
+id: r
+title: "[ R ]"
+exitN: null
+exitS: null
+exitE: null
+exitW: null
+exitU: null
+exitD: null
+items: []
+---
+
+## v2.0
+.
+## first-visit
+.
+## revisit
+.
+## examined
+.
+`
+    expect(() => parseRoom(md, 'rooms/r.md')).toThrow(/invalid section header "## v2\.0"/)
+  })
+})
diff --git a/src/world/loader.ts b/src/world/loader.ts
index 22d7cd2..75880bb 100644
--- a/src/world/loader.ts
+++ b/src/world/loader.ts
@@ -40,13 +40,26 @@ function stripWikilink(value: unknown): unknown {
 
 function splitSections(body: string): Record<string, string> {
   const sections: Record<string, string> = {}
-  // Section names use only [A-Za-z0-9_-]; headers with spaces or dots are silently skipped.
+  // First pass: detect ANY ## line that doesn't match the strict pattern.
+  // Section names use only [a-zA-Z0-9_-]; headers with spaces or other characters
+  // would silently fail without this check.
+  const looseHeader = /^##[ \t]+(.+?)[ \t]*$/gm
+  const strictHeader = /^([\w-]+)$/
+  for (const m of body.matchAll(looseHeader)) {
+    const headerText = m[1]!
+    if (!strictHeader.test(headerText)) {
+      throw new Error(
+        `invalid section header "## ${headerText}": section names must contain only letters, digits, hyphens, and underscores`,
+      )
+    }
+  }
+  // Second pass: extract sections (re-runs strict regex; same result as before).
   const re = /^##\s+([\w-]+)\s*$/gm
   const matches = [...body.matchAll(re)]
   for (let i = 0; i < matches.length; i++) {
     const m = matches[i]!
     const key = m[1]!
-    const start = m.index! + m[0]!.length
+    const start = m.index! + m[0].length
     const end = i + 1 < matches.length ? matches[i + 1]!.index! : body.length
     sections[key] = body.slice(start, end).trim()
   }