diff --git a/docs/configuration.md b/docs/configuration.md
index 094223f..ccfbf17 100644
--- a/docs/configuration.md
+++ b/docs/configuration.md
@@ -287,7 +287,8 @@ The builds still run in their branch's worktree, one build per branch at a time,
 
 `branches.<name>.autoBuild` (`enabled` + `times`) is the deprecated pre-ADR-0007 schedule, kept for compatibility: it rebuilds the branch's own pool with its regular command and logs a deprecation warning.
 `autoBuild.times` entries carrying their own `buildCommand`/`name` (a short-lived v0.9.13 syntax) are no longer supported — use a build definition.
-The concurrency limit that used to live in this section moved to `executor.maxConcurrent` without an alias — a leftover `builds.maxConcurrent` key is rejected as an invalid build definition.
+The concurrency limit that used to live in this section moved to `executor.maxConcurrent` without an alias.
+A leftover `builds.maxConcurrent` key (or any other scalar where a definition belongs) is ignored with a warning, not a startup failure — a committed config cannot always be changed right away.
 
 ### Notes on `watcher.fastForwardLocalRefs`
 
diff --git a/src/main/kotlin/de/hoennig/gittally/config/ConfigLoader.kt b/src/main/kotlin/de/hoennig/gittally/config/ConfigLoader.kt
index 93ea1d1..72f4984 100644
--- a/src/main/kotlin/de/hoennig/gittally/config/ConfigLoader.kt
+++ b/src/main/kotlin/de/hoennig/gittally/config/ConfigLoader.kt
@@ -6,19 +6,26 @@ import com.fasterxml.jackson.databind.SerializationFeature
 import com.fasterxml.jackson.dataformat.yaml.YAMLFactory
 import com.fasterxml.jackson.dataformat.yaml.YAMLGenerator
 import com.fasterxml.jackson.module.kotlin.registerKotlinModule
+import org.slf4j.LoggerFactory
 import org.springframework.stereotype.Service
 import java.io.File
 import java.nio.file.Path
 import java.nio.file.Paths
+import java.util.concurrent.ConcurrentHashMap
 
 @Service
 class ConfigLoader {
+    private val log = LoggerFactory.getLogger(ConfigLoader::class.java)
+
     private val yaml =
         ObjectMapper(YAMLFactory().disable(YAMLGenerator.Feature.WRITE_DOC_START_MARKER))
             .registerKotlinModule()
             .configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false)
             .configure(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS, false)
 
+    /** Keys already reported by [dropNonDefinitionBuilds]; the config is loaded on every poll cycle. */
+    private val warnedBuildKeys = ConcurrentHashMap.newKeySet<String>()
+
     fun load(workingDir: Path = Paths.get(".")): GitTallyConfig = toConfig(loadRaw(workingDir))
 
     /**
@@ -59,11 +66,38 @@ class ConfigLoader {
             if (raw.isEmpty()) {
                 GitTallyConfig()
             } else {
-                yaml.convertValue(mergeBranchDefaults(raw), GitTallyConfig::class.java)
+                yaml.convertValue(mergeBranchDefaults(dropNonDefinitionBuilds(raw)), GitTallyConfig::class.java)
             }
         return defaultPublicBaseUrl(config)
     }
 
+    /**
+     * Ignores `builds` entries that are not a build definition — a scalar where a
+     * definition belongs, most likely the `builds.maxConcurrent` key that moved to
+     * `executor.maxConcurrent`. Such a leftover is a warning, not a startup failure:
+     * the config lives in a repository whose `master` may not be changeable right now,
+     * and the rest of it is perfectly usable.
+     */
+    @Suppress("UNCHECKED_CAST")
+    private fun dropNonDefinitionBuilds(raw: Map<String, Any?>): Map<String, Any?> {
+        val builds = raw["builds"] as? Map<String, Any?> ?: return raw
+        val definitions = builds.filterValues { it is Map<*, *> }
+        if (definitions.size == builds.size) {
+            return raw
+        }
+        for (key in builds.keys - definitions.keys) {
+            if (!warnedBuildKeys.add(key)) {
+                continue
+            }
+            if (key == "maxConcurrent") {
+                log.warn("ignoring builds.maxConcurrent; the concurrency limit is executor.maxConcurrent since v0.9.15")
+            } else {
+                log.warn("ignoring builds.{}: a build definition must be a mapping of keys", key)
+            }
+        }
+        return raw + ("builds" to definitions)
+    }
+
     /**
      * Removes the keys a branch must never override: the secret and host-side top-level
      * sections, the per-branch trust gate, and the docker sandbox policy.
diff --git a/src/main/resources/templates/releases.html b/src/main/resources/templates/releases.html
index 504ce21..29dc999 100644
--- a/src/main/resources/templates/releases.html
+++ b/src/main/resources/templates/releases.html
@@ -23,8 +23,9 @@
             <li><strong>Changed:</strong> the build concurrency limit moved from
                 <code>builds.maxConcurrent</code> to <code>executor.maxConcurrent</code> (default 1,
                 no compatibility alias) — the <code>builds</code> section now holds build definitions
-                only. A leftover <code>builds.maxConcurrent</code> key is rejected as an invalid
-                build definition.</li>
+                only. A leftover <code>builds.maxConcurrent</code> key is ignored with a warning
+                instead of failing the configuration, so an installation keeps running until its
+                committed config can be updated.</li>
         </ul>
 
         <h2>v0.9.14 <span class="muted">— 2026-08-28</span></h2>
diff --git a/src/test/kotlin/de/hoennig/gittally/config/ConfigLoaderTest.kt b/src/test/kotlin/de/hoennig/gittally/config/ConfigLoaderTest.kt
index f27cebc..2c40364 100644
--- a/src/test/kotlin/de/hoennig/gittally/config/ConfigLoaderTest.kt
+++ b/src/test/kotlin/de/hoennig/gittally/config/ConfigLoaderTest.kt
@@ -92,6 +92,23 @@ class ConfigLoaderTest : FunSpec() {
             loader.load(dir).effectiveBuildDefinitions()["default"] shouldBe BuildDefinition(onPush = false)
         }
 
+        test("a leftover builds.maxConcurrent is ignored instead of failing the config") {
+            val dir = Files.createTempDirectory("gittally-test")
+            dir.resolve(".gittally.yml").toFile().writeText(
+                """
+                builds:
+                  maxConcurrent: 1
+                  pitest:
+                    buildCommand: ./gradlew piTestFull
+                """.trimIndent(),
+            )
+
+            val config = loader.load(dir)
+
+            config.executor.maxConcurrent shouldBe 1
+            config.buildDefinitions.keys shouldBe setOf("pitest")
+        }
+
         test("a branch may redefine the builds section for its own builds") {
             val dir = Files.createTempDirectory("gittally-test")
             dir.resolve(".gittally.yml").toFile().writeText(