I think i see why.

src/main/java/net/runelite/deob/deobfuscators/rename/MappingExecutorUtil.java

@@ -112,12 +112,26 @@ public class MappingExecutorUtil
 			mi1.map(mappings, p1, p2);
 
 			assert e.frames.size() == e2.frames.size();
+			//checkConsistency(e, e2);
 			e.paused = e2.paused = false;
 		}
 		
 		return mappings;
 	}
 	
+	private static void checkConsistency(Execution e, Execution e2)
+	{
+		assert e.frames.size() == e2.frames.size();
+		
+		for (int i = 0; i < e.frames.size(); ++i)
+		{
+			Frame f1 = e.frames.get(i), f2 = e2.frames.get(i);
+			
+			assert f1.other == f2;
+			assert f2.other == f1;
+		}
+	}
+	
 	public static boolean isMappable(InvokeInstruction ii)
 	{
 		String className;

src/main/java/net/runelite/deob/execution/ParallellMappingExecutor.java

@@ -73,7 +73,7 @@ public class ParallellMappingExecutor
 				assert fork1.getBranches().size() == 1;
 				//assert fork1.getBranches().get(0) == f1;
 				
-				int i1 = e.frames.indexOf(fork1.getFrame());
+				int i1 = e.frames.indexOf(fork1.getFrame()); // this might be -1 because it is now in an invokestatic. AHhh.
 				int i2 = e.frames.indexOf(fork1.getBranches().get(0));
 				
 				// remove fork1.frame
@@ -354,6 +354,7 @@ public class ParallellMappingExecutor
 		// replace frame with returnTo
 		assert e.frames.get(0) == f;
 		e.frames.remove(0);
+		assert !e.frames.contains(f.returnTo);
 		e.frames.add(0, f.returnTo);
 		
 		assert f.other.other == f;