Here is the error I observe, I might do something wrong:

RangeError (index): Index out of range: index should be less than 3: 3
dart:typed_data                                   Uint8List.[]
package:image/src/image/palette_uint8.dart 66:35  PaletteUint8.get
package:image/src/image/pixel_uint1.dart 166:18   PixelUint1._getChannel

The input is a simple png of 5×5 yellow pixels.

Here is the code:

Iterable<img.Image> getUniqTiles(img.Image image, int tileSize) {
  // Map to store unique tiles
  final Map<String, img.Image> uniqueTiles = {};

  // Split image into tiles
  for (int y = 0; y < image.height; y += tileSize) {
    for (int x = 0; x < image.width; x += tileSize) {
      if (y + tileSize > image.height || x + tileSize > image.width) {
        dev.log("skip too small tile");
        continue;
      }
      final img.Image tile = img.copyCrop(
        image,
        x: x,
        y: y,
        width: tileSize,
        height: tileSize,
        antialias: false,
      );

      // Convert tile to a unique key
      final tileKey = _tileToKey(tile);

      uniqueTiles[tileKey] = tile;
    }
  }
  return uniqueTiles.values;
}

String _tileToKey(img.Image tile) {
  if (!tile.isValid) {
    throw StateError("tile invalid");
  }
  if (tile.isEmpty) {
    throw StateError("tile empty");
  }
  final buffer = StringBuffer();
  for (int y = 0; y < tile.height; y++) {
    for (int x = 0; x < tile.width; x++) {
      final pixel = tile.getPixel(x, y);
      if (!pixel.isValid) {
        throw StateError("invalid pixel");
      }
      num red = pixel.r;
      num green = pixel.g;
      num blue = pixel.b;
      num alpha = pixel.a;
      buffer.write('$red,$green,$blue,$alpha;');
    }
  }
  return buffer.toString();
}

void main() {
  test('Counter value should be incremented', () async {
    img.Image? image = await img.decodePngFile('test/testData/5x5yellow.png');
    if (image == null) {
      throw StateError("bad input");
    }
    var tiles = getUniqTiles(image, 5);
    expect(tiles.length, 1);
  });
}

The error occurs on num red = pixel.r; (when I try to access the pixel value) in _tileToKey

my usage ofimg.copyCrop might be the issue.