diff --git a/forge/src/main/java/org/embeddedt/modernfix/forge/mixin/bugfix/cofh_core_crash/FlagManagerMixin.java b/forge/src/main/java/org/embeddedt/modernfix/forge/mixin/bugfix/cofh_core_crash/FlagManagerMixin.java
new file mode 100644
index 00000000..f39e12e1
--- /dev/null
+++ b/forge/src/main/java/org/embeddedt/modernfix/forge/mixin/bugfix/cofh_core_crash/FlagManagerMixin.java
@@ -0,0 +1,36 @@
+package org.embeddedt.modernfix.forge.mixin.bugfix.cofh_core_crash;
+
+import it.unimi.dsi.fastutil.objects.Object2ObjectOpenHashMap;
+import org.embeddedt.modernfix.annotation.RequiresMod;
+import org.spongepowered.asm.mixin.*;
+import org.spongepowered.asm.mixin.injection.At;
+import org.spongepowered.asm.mixin.injection.Coerce;
+import org.spongepowered.asm.mixin.injection.Redirect;
+
+import java.util.function.BooleanSupplier;
+
+/**
+ * Fix getOrCreateFlag accessing the FLAGS map without synchronization by wrapping all calls to it
+ * in a synchronized block.
+ */
+@Pseudo
+@Mixin(targets = { "cofh/lib/util/flags/FlagManager" }, remap = false)
+@RequiresMod("cofh_core")
+public class FlagManagerMixin {
+    @Shadow @Final
+    private static Object2ObjectOpenHashMap<String, BooleanSupplier> FLAGS;
+
+    @Shadow
+    private BooleanSupplier getOrCreateFlag(String flag) {
+        throw new AssertionError();
+    }
+
+    @Redirect(method = "*", at = @At(value = "INVOKE", target = "getOrCreateFlag"), require = 0)
+    private BooleanSupplier getFlag(@Coerce Object flagHandler, String flag) {
+        if(flagHandler != this)
+            throw new AssertionError("Redirect targeted bad getOrCreateFlag invocation");
+        synchronized (FLAGS) {
+            return this.getOrCreateFlag(flag);
+        }
+    }
+}