diff --git a/autograder-core/src/main/java/de/firemage/autograder/core/check/api/CommonReimplementation.java b/autograder-core/src/main/java/de/firemage/autograder/core/check/api/CommonReimplementation.java
index 6387a588..a7acee16 100644
--- a/autograder-core/src/main/java/de/firemage/autograder/core/check/api/CommonReimplementation.java
+++ b/autograder-core/src/main/java/de/firemage/autograder/core/check/api/CommonReimplementation.java
@@ -25,6 +25,7 @@
 import spoon.reflect.code.CtInvocation;
 import spoon.reflect.code.CtLiteral;
 import spoon.reflect.code.CtNewArray;
+import spoon.reflect.code.CtNewClass;
 import spoon.reflect.code.CtOperatorAssignment;
 import spoon.reflect.code.CtStatement;
 import spoon.reflect.code.CtVariableAccess;
@@ -161,6 +162,11 @@ private void checkArraysFill(CtFor ctFor) {
             return;
         }
 
+        // ignore new array or new class assignments
+        if (ctAssignment.getAssignment() instanceof CtNewClass<?> || ctAssignment.getAssignment() instanceof CtNewArray<?>) {
+            return;
+        }
+
         CtExpression<?> rhs = ctAssignment.getAssignment();
         if (!SpoonUtil.isImmutable(rhs.getType())) {
             return;
diff --git a/autograder-core/src/main/java/de/firemage/autograder/core/check/api/UseFormatString.java b/autograder-core/src/main/java/de/firemage/autograder/core/check/api/UseFormatString.java
index a5bf052c..8a22fa99 100644
--- a/autograder-core/src/main/java/de/firemage/autograder/core/check/api/UseFormatString.java
+++ b/autograder-core/src/main/java/de/firemage/autograder/core/check/api/UseFormatString.java
@@ -90,6 +90,11 @@ private String buildFormattedString(Iterable<? extends CtExpression<?>> ctExpres
                     } else {
                         formatString.append(value);
                     }
+
+                    // if the string ends with a %, the concatenation is likely used to build a format string
+                    if (value.endsWith("%")) {
+                        return null;
+                    }
                 } else if (ctTypeInformation.isPrimitive() && !ctTypeInformation.isArray()) {
                     // inline literals:
                     formatString.append(ctLiteral.getValue());
diff --git a/autograder-core/src/main/java/de/firemage/autograder/core/check/complexity/RedundantVariable.java b/autograder-core/src/main/java/de/firemage/autograder/core/check/complexity/RedundantVariable.java
index e90e9e46..243d1318 100644
--- a/autograder-core/src/main/java/de/firemage/autograder/core/check/complexity/RedundantVariable.java
+++ b/autograder-core/src/main/java/de/firemage/autograder/core/check/complexity/RedundantVariable.java
@@ -21,6 +21,8 @@
 
 @ExecutableCheck(reportedProblems = {ProblemType.REDUNDANT_VARIABLE})
 public class RedundantVariable extends IntegratedCheck {
+    private static final int MAX_EXPRESSION_SIZE = 40;
+
     /**
      * Checks if the given statement does not influence the variable returned by the return statement.
      *
@@ -32,7 +34,7 @@ private boolean isAllowedStatement(CtStatement ctStatement) {
     }
 
     private boolean isComplexExpression(CtExpression<?> ctExpression) {
-        return ctExpression instanceof CtSwitchExpression<?,?>;
+        return ctExpression instanceof CtSwitchExpression<?,?> || ctExpression.toString().length() > MAX_EXPRESSION_SIZE;
     }
 
     private void checkVariableRead(CtStatement ctStatement, CtVariableRead<?> ctVariableRead) {
diff --git a/autograder-core/src/main/java/de/firemage/autograder/core/check/naming/LinguisticNamingCheck.java b/autograder-core/src/main/java/de/firemage/autograder/core/check/naming/LinguisticNamingCheck.java
index 86088d4f..5dccb2e4 100644
--- a/autograder-core/src/main/java/de/firemage/autograder/core/check/naming/LinguisticNamingCheck.java
+++ b/autograder-core/src/main/java/de/firemage/autograder/core/check/naming/LinguisticNamingCheck.java
@@ -94,7 +94,7 @@ private <T> void checkCtMethod(CtMethod<T> ctMethod) {
             return;
         }
 
-        if (prefix.equals("set") && isInvalidSetterReturnType(ctMethod)) {
+        if (prefix.equals("set") && isInvalidSetterReturnType(ctMethod) && SpoonUtil.getEffectiveStatements(ctMethod.getBody()).size() <= 3) {
             // it is expected that a setter returns nothing (void)
             this.reportProblem(
                 "linguistic-naming-setter",
diff --git a/autograder-core/src/main/java/de/firemage/autograder/core/check/oop/InheritanceBadPractices.java b/autograder-core/src/main/java/de/firemage/autograder/core/check/oop/InheritanceBadPractices.java
index 2d9b9644..132fb6c9 100644
--- a/autograder-core/src/main/java/de/firemage/autograder/core/check/oop/InheritanceBadPractices.java
+++ b/autograder-core/src/main/java/de/firemage/autograder/core/check/oop/InheritanceBadPractices.java
@@ -48,14 +48,14 @@ public void process(CtClass<?> ctClass) {
                 }
 
                 // check if the class can be an interface:
-                if (fields.isEmpty()) {
+                if (fields.isEmpty() && ctClass.getSuperclass() == null) {
                     addLocalProblem(
                         ctClass,
                         new LocalizedMessage("should-be-interface"),
                         ProblemType.SHOULD_BE_INTERFACE
                     );
                     // check if the class has only fields (data class)
-                } else if (methods.isEmpty()) {
+                } else if (methods.isEmpty() && ctClass.getSuperclass() == null) {
                     String methodName = IdentifierNameUtils.toLowerCamelCase(ctClass.getSimpleName());
                     addLocalProblem(
                         ctClass,
diff --git a/autograder-core/src/test/java/de/firemage/autograder/core/check/api/TestUseFormatString.java b/autograder-core/src/test/java/de/firemage/autograder/core/check/api/TestUseFormatString.java
index 6a51ce94..695cdf33 100644
--- a/autograder-core/src/test/java/de/firemage/autograder/core/check/api/TestUseFormatString.java
+++ b/autograder-core/src/test/java/de/firemage/autograder/core/check/api/TestUseFormatString.java
@@ -126,4 +126,26 @@ public String toString() {
 
         problems.assertExhausted();
     }
+
+    @Test
+    void testBuildFormatString() throws LinterException, IOException {
+        ProblemIterator problems = this.checkIterator(StringSourceInfo.fromSourceString(
+            JavaVersion.JAVA_17,
+            "Field",
+            """
+                public class Field {
+                    private String left;
+                    private int number;
+                    private String right;
+
+                    @Override
+                    public String toString() {
+                        return String.format("output: " + "%" + this.number + "d", this.number);
+                    }
+                }
+                """
+        ), PROBLEM_TYPES);
+
+        problems.assertExhausted();
+    }
 }