diff --git a/examples/hello_compute_rust/main.rs b/examples/hello_compute_rust/main.rs
index 27c5ad7746..bac59cf1c1 100644
--- a/examples/hello_compute_rust/main.rs
+++ b/examples/hello_compute_rust/main.rs
@@ -94,10 +94,10 @@ fn main() {
         if let wgpu::BufferMapAsyncResult::Success(data) = result {
             println!("Times: {:?}", data);
         }
-
-        // staging_buffer.unmap(); // TODO
     });
 
     let encoder = device.create_command_encoder(&wgpu::CommandEncoderDescriptor { todo: 0 });
     device.get_queue().submit(&[encoder.finish()]);
+
+    staging_buffer.unmap(); // TODO: staging_buffer can't be referenced from the callback
 }
diff --git a/wgpu-native/src/device.rs b/wgpu-native/src/device.rs
index b1cab11580..9d466ecc8c 100644
--- a/wgpu-native/src/device.rs
+++ b/wgpu-native/src/device.rs
@@ -1059,11 +1059,15 @@ pub extern "C" fn wgpu_queue_submit(
 
         let mut swap_chain_links = Vec::new();
 
+        let mut trackers = device.trackers.lock();
+        let mut destroyed = device.destroyed.lock();
+        destroyed.triage_referenced(&mut *trackers);
+        destroyed.triage_framebuffers(&mut *device.framebuffers.lock());
+
         let old_submit_index = device
             .life_guard
             .submission_index
             .fetch_add(1, Ordering::Relaxed);
-        let mut trackers = device.trackers.lock();
 
         //TODO: if multiple command buffers are submitted, we can re-use the last
         // native command buffer of the previous chain instead of always creating
@@ -1170,12 +1174,8 @@ pub extern "C" fn wgpu_queue_submit(
     let device_guard = HUB.devices.read();
     let device = &device_guard[queue_id];
 
-    let mut trackers = device.trackers.lock();
-
     let last_done = {
         let mut destroyed = device.destroyed.lock();
-        destroyed.triage_referenced(&mut *trackers);
-        destroyed.triage_framebuffers(&mut *device.framebuffers.lock());
         let last_done = destroyed.cleanup(&device.raw);
         destroyed.handle_mapping(&device.raw);
 
@@ -1776,6 +1776,12 @@ pub extern "C" fn wgpu_buffer_map_read_async(
     let mut buffer_guard = HUB.buffers.write();
     let buffer = &mut buffer_guard[buffer_id];
 
+    if buffer.pending_map_operation.is_some() {
+        log::error!("wgpu_buffer_map_read_async failed: buffer mapping is pending");
+        callback(BufferMapAsyncStatus::Error, std::ptr::null_mut(), userdata);
+        return;
+    }
+
     let range = start as u64..(start + size) as u64;
     buffer.pending_map_operation = Some(BufferMapOperation::Read(range, callback, userdata));
 
@@ -1796,6 +1802,12 @@ pub extern "C" fn wgpu_buffer_map_write_async(
     let mut buffer_guard = HUB.buffers.write();
     let buffer = &mut buffer_guard[buffer_id];
 
+    if buffer.pending_map_operation.is_some() {
+        log::error!("wgpu_buffer_map_write_async failed: buffer mapping is pending");
+        callback(BufferMapAsyncStatus::Error, std::ptr::null_mut(), userdata);
+        return;
+    }
+
     let range = start as u64..(start + size) as u64;
     buffer.pending_map_operation = Some(BufferMapOperation::Write(range, callback, userdata));