diff --git a/YesNt.Interpreter.Tests/CodeFlowTests.cs b/YesNt.Interpreter.Tests/CodeFlowTests.cs
index a3c0527..ff52902 100644
--- a/YesNt.Interpreter.Tests/CodeFlowTests.cs
+++ b/YesNt.Interpreter.Tests/CodeFlowTests.cs
@@ -200,6 +200,24 @@ public class CodeFlowTests
         YesNtAssert.IsLastLineEqual(lines, "ok");
     }
 
+    [TestMethod]
+    public void GotoInsideLoopExitsLoopTest()
+    {
+        List<string> lines =
+        [
+            "var hit = no",
+            "while 1 == 1:",
+            "var hit = yes",
+            "goto done",
+            "end_while",
+            "label done:",
+            "${hit}"
+        ];
+
+        YesNtAssert.IsLastLineEqual(lines, "yes");
+    }
+
+
     [TestMethod]
     public void LabelWithoutColonFailsTest()
     {
diff --git a/YesNt.Interpreter.Tests/FunctionStatementsTests.cs b/YesNt.Interpreter.Tests/FunctionStatementsTests.cs
index aa148b9..134b6e3 100644
--- a/YesNt.Interpreter.Tests/FunctionStatementsTests.cs
+++ b/YesNt.Interpreter.Tests/FunctionStatementsTests.cs
@@ -132,6 +132,25 @@ public class FunctionStatementsTests
         YesNtAssert.ContainsTerminationMessage(lines, "Nested functions are not allowed");
     }
 
+    [TestMethod]
+    public void LocalVariableDoesNotLeakToCallerTest()
+    {
+        List<string> lines =
+        [
+            "goto main",
+            "func modify:",
+            "var x = inner",
+            "return",
+            "label main:",
+            "var x = outer",
+            "call modify",
+            "${x}"
+        ];
+
+        YesNtAssert.IsLastLineEqual(lines, "outer");
+    }
+
+
     [TestMethod]
     public void ClearCallStackRunsTest()
     {
@@ -144,5 +163,66 @@ public class FunctionStatementsTests
 
         YesNtAssert.IsLastLineEqual(lines, "ok");
     }
+
+    // --- Error path tests ---
+
+    [TestMethod]
+    public void AccessInWithoutArgFailsTest()
+    {
+        List<string> lines =
+        [
+            "goto main",
+            "func noin:",
+            "var x = %in",
+            "return",
+            "label main:",
+            "call noin"
+        ];
+
+        YesNtAssert.ContainsTerminationMessage(lines, "No in argument in stack");
+    }
+
+    // --- Nested scope tests ---
+
+    [TestMethod]
+    public void GlobalModifiedInsideFunctionIsVisibleAfterReturnTest()
+    {
+        List<string> lines =
+        [
+            "goto main",
+            "func setglobal:",
+            "global shared = modified",
+            "return",
+            "label main:",
+            "global shared = original",
+            "call setglobal",
+            "${shared}"
+        ];
+
+        YesNtAssert.IsLastLineEqual(lines, "modified");
+    }
+
+    [TestMethod]
+    public void NestedFunctionCallsHaveIndependentLocalScopesTest()
+    {
+        List<string> lines =
+        [
+            "goto main",
+            "func outer:",
+            "var x = outer_val",
+            "call inner",
+            "push_out ${x}",
+            "return",
+            "func inner:",
+            "var x = inner_val",
+            "return",
+            "label main:",
+            "call outer",
+            "var result = %out",
+            "${result}"
+        ];
+
+        YesNtAssert.IsLastLineEqual(lines, "outer_val");
+    }
 }
 
diff --git a/YesNt.Interpreter.Tests/ProcessingStatementsTests.cs b/YesNt.Interpreter.Tests/ProcessingStatementsTests.cs
index 4d20081..c845d2c 100644
--- a/YesNt.Interpreter.Tests/ProcessingStatementsTests.cs
+++ b/YesNt.Interpreter.Tests/ProcessingStatementsTests.cs
@@ -63,6 +63,32 @@ public class ProcessingStatementsTests
         YesNtAssert.IsLastLineEqual(lines, "hello\nworld");
     }
 
+    [TestMethod]
+    public void CalcRespectsPrecedenceTest()
+    {
+        YesNtAssert.IsLineEqual("2 + 3 * 4 calc", "14");
+    }
+
+    [TestMethod]
+    public void CalcParenthesesOverridePrecedenceTest()
+    {
+        YesNtAssert.IsLineEqual("(2 + 3) * 4 calc", "20");
+    }
+
+    [TestMethod]
+    public void SleepZeroIsValidTest()
+    {
+        List<string> lines =
+        [
+            "sleep 0",
+            "var result = ok",
+            "${result}"
+        ];
+
+        YesNtAssert.IsLastLineEqual(lines, "ok");
+    }
+
+
     [TestMethod]
     public void SleepInvalidValueFailsTest()
     {
@@ -152,5 +178,26 @@ public class ProcessingStatementsTests
 
         YesNtAssert.IsLastLineEqual(lines, "1", timeout: 3000);
     }
+
+    [TestMethod]
+    public void MultipleTasksRunConcurrentlyTest()
+    {
+        List<string> lines =
+        [
+            "global a = 0",
+            "global b = 0",
+            "global a = 1 task",
+            "global b = 2 task",
+            "while ${a} == 0:",
+            "sleep 10",
+            "end_while",
+            "while ${b} == 0:",
+            "sleep 10",
+            "end_while",
+            "${b}"
+        ];
+
+        YesNtAssert.IsLastLineEqual(lines, "2", timeout: 3000);
+    }
 }
 
diff --git a/YesNt.Interpreter.Tests/StringLiteralStatementsTests.cs b/YesNt.Interpreter.Tests/StringLiteralStatementsTests.cs
index e6ffcda..5f623f2 100644
--- a/YesNt.Interpreter.Tests/StringLiteralStatementsTests.cs
+++ b/YesNt.Interpreter.Tests/StringLiteralStatementsTests.cs
@@ -58,6 +58,48 @@ public class StringLiteralStatementsTests
         YesNtAssert.IsLastLineEqual(lines, "hello world");
     }
 
+    [TestMethod]
+    public void EmptyStringLiteralHasLengthZeroTest()
+    {
+        List<string> lines =
+        [
+            "var x = \"\"",
+            "length ${x}",
+            "var len = %out",
+            "${len}"
+        ];
+
+        YesNtAssert.IsLastLineEqual(lines, "0");
+    }
+
+    [TestMethod]
+    public void EscapedBackslashProducesLiteralBackslashTest()
+    {
+        List<string> lines =
+        [
+            "var msg = \"\\\\n\"",   // YesNt source: "\\n" → \n (backslash + n, 2 chars)
+            "length ${msg}",
+            "var len = %out",
+            "${len}"
+        ];
+
+        YesNtAssert.IsLastLineEqual(lines, "2");
+    }
+
+    [TestMethod]
+    public void UnknownEscapeSequenceBecomesCharTest()
+    {
+        List<string> lines =
+        [
+            "var msg = \"\\q\"",   // YesNt source: "\q" → q (1 char)
+            "length ${msg}",
+            "var len = %out",
+            "${len}"
+        ];
+
+        YesNtAssert.IsLastLineEqual(lines, "1");
+    }
+
     [TestMethod]
     public void UnterminatedStringLiteralFailsTest()
     {
diff --git a/YesNt.Interpreter.Tests/VariableStatementsTests.cs b/YesNt.Interpreter.Tests/VariableStatementsTests.cs
index d7e07e3..9bf890b 100644
--- a/YesNt.Interpreter.Tests/VariableStatementsTests.cs
+++ b/YesNt.Interpreter.Tests/VariableStatementsTests.cs
@@ -91,6 +91,34 @@ public class VariableStatementsTests
         YesNtAssert.ContainsTerminationMessage(lines, "Invalid syntax");
     }
 
+    [TestMethod]
+    public void OverwriteVariableTest()
+    {
+        List<string> lines =
+        [
+            "var x = first",
+            "var x = second",
+            "${x}"
+        ];
+
+        YesNtAssert.IsLastLineEqual(lines, "second");
+    }
+
+    [TestMethod]
+    public void EmptyStringVariableHasLengthZeroTest()
+    {
+        List<string> lines =
+        [
+            "var x = \"\"",
+            "length ${x}",
+            "var len = %out",
+            "${len}"
+        ];
+
+        YesNtAssert.IsLastLineEqual(lines, "0");
+    }
+
+
     [TestMethod]
     public void MissingVariableReferenceFailsTest()
     {