I think i see why.

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

@@ -84,6 +84,8 @@ public class MappingExecutorUtil
 		
 		while (parallel.step())
 		{
+			assert e.frames.size() == e2.frames.size();
+			
 			// get what each frame is paused/exited on
 			InstructionContext p1 = parallel.getP1(), p2 = parallel.getP2();
 			
@@ -109,6 +111,7 @@ public class MappingExecutorUtil
 
 			mi1.map(mappings, p1, p2);
 
+			assert e.frames.size() == e2.frames.size();
 			e.paused = e2.paused = false;
 		}
 		

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

@@ -53,7 +53,7 @@ public class Frame
 	@Override
 	public String toString()
 	{
-		return "Frame{" + "cur=" + cur + '}';
+		return "Frame{" + "cur=" + cur + ", last=" + this.lastInstruction() + "}";
 	}
 	
 	public void initialize()
@@ -253,6 +253,11 @@ public class Frame
 		cur = instructions.get(idx + 1);
 	}
 	
+	private InstructionContext lastInstruction()
+	{
+		return instructions.isEmpty() ? null : instructions.get(instructions.size() - 1);
+	}
+	
 	private void processExceptions(Instruction i)
 	{
 		if (this.execution.step)

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

@@ -19,8 +19,16 @@ public class ParallellMappingExecutor
 	}
 	
 	boolean step1 = true, step2 = true;
+	int count;
 	public boolean step()
 	{
+		++count;
+		
+
+		if (count == 685)
+		{
+			int i = 5;
+		}
 		// this no longer holds with recursive stepinfo
 		//assert e.frames.size() == e2.frames.size();
 		
@@ -47,26 +55,28 @@ public class ParallellMappingExecutor
 			assert f1.returnTo == null || !e.frames.contains(f1.returnTo);
 			assert f2.returnTo == null || !e2.frames.contains(f2.returnTo);
 			
-			// I dont know if this is necessary.
+			// Due to jump ob one side can stop while the other side jumps
-//			if (f1.getInstructions().size() > 0)
+			if (f1.getInstructions().size() > 0)
-//			{
+			{
-//				p1 = f1.getInstructions().get(f1.getInstructions().size() - 1);
+				p1 = f1.getInstructions().get(f1.getInstructions().size() - 1);
-//
+
-//				for (Frame branch : p1.getBranches())
+				for (Frame branch : p1.getBranches())
-//				{
+				{
-//					e.frames.remove(branch);
+					e.frames.remove(branch);
-//				}
+				}
-//			}
+			}
-//
+
-//			if (f2.getInstructions().size() > 0)
+			// this is empty but should be removing a branch, because of the map other, theres no prev instruction.
-//			{
+			// should always populate prev instruction
-//				p2 = f2.getInstructions().get(f2.getInstructions().size() - 1);
+			if (f2.getInstructions().size() > 0)
-//				
+			{
-//				for (Frame branch : p2.getBranches())
+				p2 = f2.getInstructions().get(f2.getInstructions().size() - 1);
-//				{
+				
-//					e2.frames.remove(branch);
+				for (Frame branch : p2.getBranches())
-//				}
+				{
-//			}
+					e2.frames.remove(branch);
+				}
+			}
 			
 			assert e.frames.get(0) == f1;
 			assert e2.frames.get(0) == f2;
@@ -96,7 +106,11 @@ public class ParallellMappingExecutor
 //				assert false;
 //			}
 
-			Frame f1wtf = e.frames.get(0), f2wtf = e2.frames.get(0);
+			Frame f1wtf = e.frames.get(0),
+				f2wtf = e2.frames.get(0);
+			
+			int otherIndex1 = e2.frames.indexOf(f1wtf.other),
+				otherIndex2 = e.frames.indexOf(f2wtf.other);
 
 	//		assert f1wtf.other.other == f1wtf;
 	//		assert f2wtf.other.other == f2wtf;
@@ -104,11 +118,13 @@ public class ParallellMappingExecutor
 			assert f1wtf.other == f2wtf;
 			assert f2wtf.other == f1wtf;
 			
+			step1 = step2 = true;
+			
 			return step();
 		}
 		
 		Frame old1 = new Frame(f1), old2 = new Frame(f2);
-		int s1 = e.frames.size(), s2 = e.frames.size();
+		int s1 = e.frames.size(), s2 = e2.frames.size();
 
 		// step frame
 		if (step1)
@@ -121,16 +137,25 @@ public class ParallellMappingExecutor
 		else
 			step2 = true;
 		
-		if (e.frames.size() - s1 != e2.frames.size() - s2)
-		{
-			System.out.println("fr mismatch");
-		}
-		
 		Frame oldf1 = f1, oldf2 = f2;
 		
 		f1 = popStack(f1);
 		f2 = popStack(f2);
 		
+//		if (e.frames.size() - s1 != e2.frames.size() - s2)
+//		{
+//			System.out.println("fr mismatch");
+//		}
+		
+		if (oldf1 != f1 || oldf2 != f2)
+		{
+			if (f1 == oldf1)
+				step1 = false;
+			if (f2 == oldf2)
+				step2 = false;
+			return step();
+		}
+		
 		if (oldf1 != f1 || oldf2 != f2)
 		{
 //			assert oldf1 != f1;
@@ -157,6 +182,11 @@ public class ParallellMappingExecutor
 			return step();
 		}
 		
+		if (!(e.frames.size() == e2.frames.size()))
+		{
+			int i =56;
+		}
+		
 		if (MappingExecutorUtil.isInlineable(p1.getInstruction()) && !MappingExecutorUtil.isInlineable(p2.getInstruction()))
 		{
 			f1 = stepInto(f1);
@@ -197,6 +227,11 @@ public class ParallellMappingExecutor
 			
 			return step();
 		}
+		
+		if (!(e.frames.size() == e2.frames.size()))
+		{
+			int i =56;
+		}
 
 		assert e.paused;
 		assert e2.paused;
@@ -229,7 +264,7 @@ public class ParallellMappingExecutor
 		
 		assert methods.size() == 1;
 		
-		Method to = is.getMethods().get(0);
+		Method to = methods.get(0);
 		
 		Frame f2 = new Frame(e, to);
 		f2.initialize(i);
@@ -263,7 +298,7 @@ public class ParallellMappingExecutor
 		f = popStackForce(f);
 		
 		// step return frame
-		f.execute();
+		//f.execute();
 		
 		return f;
 	}