diff --git a/crates/pecos-qasm/src/engine.rs b/crates/pecos-qasm/src/engine.rs
index 63f70f4e8..64c27289d 100644
--- a/crates/pecos-qasm/src/engine.rs
+++ b/crates/pecos-qasm/src/engine.rs
@@ -1238,15 +1238,16 @@ impl QASMEngine {
         match name {
             "RNGseed" => {
                 if args.len() != 1 {
-                    return Err(PecosError::ParseInvalidExpression(
-                        "Expected a single seed for RNGseed. Received {args.len()}".to_string(),
-                    ));
+                    return Err(PecosError::ParseInvalidExpression(format!(
+                        "RNGseed expects exactly 1 argument (seed), got {}",
+                        args.len()
+                    )));
                 }
                 let seed: u64 = match &args[0] {
                     Expression::Integer(bit_vec) => bit_vec.load(),
                     _ => {
                         return Err(PecosError::ParseInvalidExpression(
-                            "Invalid seed for RNGseed. Expected u64".to_string(),
+                            "RNGseed expects a u64 as its argument".to_string(),
                         ));
                     }
                 };
@@ -1256,15 +1257,16 @@ impl QASMEngine {
             }
             "RNGindex" => {
                 if args.len() != 1 {
-                    return Err(PecosError::ParseInvalidExpression(
-                        "Expected a single index for RNGseed. Received {args.len()}".to_string(),
-                    ));
+                    return Err(PecosError::ParseInvalidExpression(format!(
+                        "RNGindex expects exactly 1 argument (index), got {}",
+                        args.len()
+                    )));
                 }
                 let idx: u64 = match &args[0] {
                     Expression::Integer(bit_vec) => bit_vec.load(),
                     _ => {
                         return Err(PecosError::ParseInvalidExpression(
-                            "Invalid idx for RNGindex. Expected u64".to_string(),
+                            "RNGindex expects a u64 as its argument".to_string(),
                         ));
                     }
                 };
@@ -1273,15 +1275,16 @@ impl QASMEngine {
             }
             "RNGbound" => {
                 if args.len() != 1 {
-                    return Err(PecosError::ParseInvalidExpression(
-                        "Expected a single bound for RNGbound. Received {args.len()}".to_string(),
-                    ));
+                    return Err(PecosError::ParseInvalidExpression(format!(
+                        "RNGbound expects exactly 1 argument (bound), got {}",
+                        args.len()
+                    )));
                 }
                 let ubound: u32 = match &args[0] {
                     Expression::Integer(bit_vec) => bit_vec.load(),
                     _ => {
                         return Err(PecosError::ParseInvalidExpression(
-                            "Invalid idx for RNGindex. Expected u64".to_string(),
+                            "RNGbound expects a u32 as its argument".to_string(),
                         ));
                     }
                 };
@@ -1290,10 +1293,12 @@ impl QASMEngine {
             }
             "RNGnum" => {
                 if !args.is_empty() {
-                    return Err(PecosError::ParseInvalidExpression(
-                        "RNGnum receives no arguments. Received {args.len()}".to_string(),
-                    ));
+                    return Err(PecosError::ParseInvalidExpression(format!(
+                        "RNGnum expects no arguments, got {}",
+                        args.len()
+                    )));
                 }
+
                 let rng_num = self.rng_model.rng_num();
 
                 // convert random number to bitvec
@@ -1303,9 +1308,9 @@ impl QASMEngine {
                 }
                 Ok(ExpressionValue::BitVec(bitvec))
             }
-            _ => Err(PecosError::ParseInvalidExpression(
-                "Invalid RNG function '{name}'".to_string(),
-            )),
+            _ => Err(PecosError::ParseInvalidExpression(format!(
+                "Unknown RNG function '{name}'"
+            ))),
         }
     }
 
diff --git a/python/quantum-pecos/src/pecos/engines/cvm/rng_model.py b/python/quantum-pecos/src/pecos/engines/cvm/rng_model.py
index 612056c15..5ce26db4d 100644
--- a/python/quantum-pecos/src/pecos/engines/cvm/rng_model.py
+++ b/python/quantum-pecos/src/pecos/engines/cvm/rng_model.py
@@ -31,7 +31,7 @@ def __init__(
 
     def __str__(self) -> str:
         """Returns the str representation of the model."""
-        return f"RNG Model with bound {self.current_bound} with count {self.count}"
+        return f"RNG Model bounded by {self.current_bound} with current count {self.count}"
 
     def set_seed(self, seed: int) -> None:
         """Setting the seed for generating random numbers."""
@@ -54,8 +54,8 @@ def set_index(self, index: int) -> None:
         The number after from the stream will be the idx of interest.
         """
         if self.count > index:
-            error_msg = "rngindex called after specified already generated"
-            raise BufferError(error_msg)
+            error_msg = f"RNGindex({index}) cannot move backward: current stream index is {self.count}"
+            raise ValueError(error_msg)
         while self.count < index:
             self.rng_random()
 
@@ -97,5 +97,5 @@ def eval_func(self, params: dict, output: dict) -> None:
             binary_val = BitUInt(creg.size, rng)
             creg.set(binary_val)
         else:
-            error_msg = f"RNG function not supported {func_name}"
+            error_msg = f"Unknown RNG Function '{func_name}'"
             raise ValueError(error_msg)
diff --git a/python/quantum-pecos/tests/pecos/unit/test_rng.py b/python/quantum-pecos/tests/pecos/unit/test_rng.py
index 8fcca0ec8..7c49c0986 100644
--- a/python/quantum-pecos/tests/pecos/unit/test_rng.py
+++ b/python/quantum-pecos/tests/pecos/unit/test_rng.py
@@ -33,6 +33,20 @@ def test_bounded_random() -> None:
     assert 0 <= random_number < bound
 
 
+def test_set_idx_raises_for_backwards_index() -> None:
+    """Verifies that an error is raised when specifying an index that was already consumed in the RNG stream."""
+    rng = RNGModel(shot_id=0)
+    rng.set_seed(42)
+
+    rng.set_index(4)
+
+    try:
+        rng.set_index(3)
+    except ValueError as exc:
+        expected_error_msg = "RNGindex(3) cannot move backward: current stream index is 4"
+        assert str(exc) == expected_error_msg
+
+
 def test_set_idx() -> None:
     """Verifies that the idx is set properly for our model."""
     rng = RNGModel(shot_id=0)