Express and nginx net::ERR_CONTENT_LENGTH_MISMATCH

I'm developing an Express-driven site, that is going through an nginx proxy. Sometimes when loading a page in the browser, I get this:

GET http://myapp.local/css/bootstrap.css net::ERR_CONTENT_LENGTH_MISMATCH

If I refresh the page, it usually goes away. But if refresh over and over and over, it will come up again.

What is the problem here? What can I do to narrow down the issue here? Here is my nginx conf for this server:

server {
  listen 80;
  server_name www.myapp.local;
  rewrite ^(.*) http://myapp.local$1 permanent;
}

server {
  listen 80;
  server_name myapp.local;

  access_log /vagrant/nginx/logs/myapp.local/access.log;
  error_log /vagrant/nginx/logs/myapp.local/error.log;

  location / {
    proxy_pass http://localhost:8080;
    proxy_http_version 1.1;
    proxy_set_header Upgrade $http_upgrade;
    proxy_set_header Coection 'upgrade';
    proxy_set_header Host $host;
    proxy_cache_bypass $http_upgrade;
  }
}