Off by one...

src/main.rs

@@ -119,7 +119,7 @@ fn image_to_offimage_buffer(image: GrayAlphaImage) -> eyre::Result<Box<[u8; IOCT
     );
 
     // Iterate over pixels and copy them into `buf`.
-    for (i, pixel) in (0..buf.len()).rev().zip(image.chunks_exact(2)) {
+    for (i, pixel) in (0..buf.len() - 1).rev().zip(image.chunks_exact(2)) {
         let &[gray, alpha] = pixel else {
             unreachable!()
         };