diff --git a/commonUI/src/main/kotlin/ch/dissem/yaep/ui/common/App.kt b/commonUI/src/main/kotlin/ch/dissem/yaep/ui/common/App.kt
index 6a75278..ead7de9 100644
--- a/commonUI/src/main/kotlin/ch/dissem/yaep/ui/common/App.kt
+++ b/commonUI/src/main/kotlin/ch/dissem/yaep/ui/common/App.kt
@@ -1,15 +1,19 @@
 package ch.dissem.yaep.ui.common
 
+import androidx.compose.foundation.BorderStroke
 import androidx.compose.foundation.Image
 import androidx.compose.foundation.clickable
 import androidx.compose.foundation.layout.*
 import androidx.compose.foundation.lazy.grid.GridCells
 import androidx.compose.foundation.lazy.grid.LazyVerticalGrid
+import androidx.compose.material3.CardDefaults
 import androidx.compose.material3.HorizontalDivider
+import androidx.compose.material3.MaterialTheme
 import androidx.compose.material3.OutlinedCard
 import androidx.compose.runtime.*
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.draw.alpha
+import androidx.compose.ui.draw.shadow
 import androidx.compose.ui.unit.dp
 import ch.dissem.yaep.domain.*
 import org.jetbrains.compose.resources.painterResource
@@ -18,16 +22,31 @@ import yaep.commonui.generated.resources.neighbour
 import yaep.commonui.generated.resources.order
 
 class DisplayClue<C : Clue>(val clue: C) {
-    var isActive by mutableStateOf(true)
+    var isActive: Boolean by mutableStateOf(true)
+
+    var isViolated: Boolean by mutableStateOf(false)
+
+    fun update(grid: Grid) {
+        isViolated = !clue.isValid(grid)
+        if (isViolated) {
+            isActive = true
+        }
+    }
 }
 
 @Composable
-fun App(modifier: Modifier = Modifier) {
-    val game = remember { generateGame() }
+fun App(modifier: Modifier = Modifier, game: Game = remember { generateGame() }) {
     val horizontalClues = remember { game.horizontalClues.map { DisplayClue(it) } }
     val verticalClues = remember { game.verticalClues.map { DisplayClue(it) } }
     Row(modifier = modifier) {
-        PuzzleGrid(modifier = Modifier.aspectRatio(1f).weight(0.6f).fillMaxHeight(), game.grid)
+        PuzzleGrid(
+            modifier = Modifier.aspectRatio(1f).weight(0.6f).fillMaxHeight(),
+            grid = game.grid,
+            onUpdate = {
+                horizontalClues.forEach { it.update(game.grid) }
+                verticalClues.forEach { it.update(game.grid) }
+            }
+        )
         PuzzleClues(
             modifier = Modifier.padding(start = 16.dp).weight(0.4f).fillMaxHeight(),
             horizontalClues,
@@ -39,7 +58,8 @@ fun App(modifier: Modifier = Modifier) {
 @Composable
 fun PuzzleGrid(
     modifier: Modifier = Modifier,
-    grid: Grid
+    grid: Grid,
+    onUpdate: () -> Unit
 ) {
     Column(modifier = modifier) {
         for (row in grid) {
@@ -60,6 +80,7 @@ fun PuzzleGrid(
                         }
                         item.selectionChangedListeners.add {
                             selection = it
+                            onUpdate()
                         }
                     }
                     Selector(
@@ -94,10 +115,9 @@ fun PuzzleClues(
                 item {
                     HorizontalClue(
                         modifier = Modifier
-                            .alpha(if (clue.isActive) 1f else 0.2f)
-                            .padding(8.dp)
-                            .clickable { clue.isActive = false },
-                        clue = clue.clue
+                            .forClue(clue),
+                        clue = clue.clue,
+                        isClueViolated = clue.isViolated
                     )
                 }
             }
@@ -111,11 +131,10 @@ fun PuzzleClues(
                 item {
                     VerticalClue(
                         modifier = Modifier
-                            .alpha(if (clue.isActive) 1f else 0.2f)
-                            .padding(8.dp)
-                            .aspectRatio(0.33333334f)
-                            .clickable { clue.isActive = false },
-                        clue = clue.clue
+                            .forClue(clue)
+                            .aspectRatio(0.33333334f),
+                        clue = clue.clue,
+                        isClueViolated = clue.isViolated
                     )
                 }
             }
@@ -123,9 +142,17 @@ fun PuzzleClues(
     }
 }
 
+private fun Modifier.forClue(clue: DisplayClue<out Clue>) = this
+    .alpha(if (clue.isActive) 1f else 0.2f)
+    .padding(8.dp)
+    .clickable { clue.isActive = false }
+
 @Composable
-fun HorizontalClue(modifier: Modifier = Modifier, clue: HorizontalClue) {
-    OutlinedCard(modifier) {
+fun HorizontalClue(modifier: Modifier = Modifier, clue: HorizontalClue, isClueViolated: Boolean) {
+    ClueCard(
+        modifier = modifier,
+        isClueViolated = isClueViolated
+    ) {
         Row {
             when (clue) {
                 is NeighbourClue<*, *> -> {
@@ -159,11 +186,72 @@ fun HorizontalClue(modifier: Modifier = Modifier, clue: HorizontalClue) {
 }
 
 @Composable
-fun VerticalClue(modifier: Modifier = Modifier, clue: SameColumnClue<*, *>) {
-    OutlinedCard(modifier.aspectRatio(0.5f)) {
+fun VerticalClue(modifier: Modifier = Modifier, clue: SameColumnClue<*, *>, isClueViolated: Boolean = false) {
+    ClueCard(
+        modifier = modifier.aspectRatio(0.5f),
+        isClueViolated = isClueViolated
+    ) {
         Column {
             DrawItem(modifier = Modifier.weight(1f), clue.a)
             DrawItem(modifier = Modifier.weight(1f), clue.b)
         }
     }
 }
+
+@Composable
+fun ClueCard(modifier: Modifier = Modifier, isClueViolated: Boolean, content: @Composable () -> Unit) {
+    val colors = MaterialTheme.colorScheme
+    OutlinedCard(
+        modifier = if (isClueViolated) {
+            modifier.shadow(8.dp, shape = CardDefaults.outlinedShape, ambientColor = colors.error, spotColor = colors.error)
+        } else {
+            modifier
+        },
+        border = if (isClueViolated) {
+            remember { BorderStroke(1.0.dp, colors.error) }
+        } else {
+            CardDefaults.outlinedCardBorder()
+        }
+    ) {
+        content()
+    }
+}
+
+//fun Modifier.advanceShadow(
+//    color: Color = Color.Black,
+//    borderRadius: Dp = 16.dp,
+//    blurRadius: Dp = 16.dp,
+//    offsetY: Dp = 0.dp,
+//    offsetX: Dp = 0.dp,
+//    spread: Float = 1f,
+//) = drawBehind {
+//    this.drawIntoCanvas {
+//        val paint = Paint()
+//        val frameworkPaint = paint.asFrameworkPaint()
+//        val spreadPixel = spread.dp.toPx()
+//        val leftPixel = (0f - spreadPixel) + offsetX.toPx()
+//        val topPixel = (0f - spreadPixel) + offsetY.toPx()
+//        val rightPixel = (this.size.width)
+//        val bottomPixel = (this.size.height + spreadPixel)
+//
+//        if (blurRadius != 0.dp) {
+//            /*
+//                The feature maskFilter used below to apply the blur effect only works
+//                with hardware acceleration disabled.
+//             */
+//            frameworkPaint.maskFilter =
+//                BlurEffect(blurRadius.toPx(), blurRadius.toPx())
+//        }
+//
+//        frameworkPaint.color = color.toArgb()
+//        it.drawRoundRect(
+//            left = leftPixel,
+//            top = topPixel,
+//            right = rightPixel,
+//            bottom = bottomPixel,
+//            radiusX = borderRadius.toPx(),
+//            radiusY = borderRadius.toPx(),
+//            paint
+//        )
+//    }
+//}