The issue is easily reproduced with the following command: $ ab -n100 -c100 http://localhost/js/app.js

I typically see 1 successful request and 99 failures due to net::ERR_CONTENT_LENGTH_MISMATCH.

The js file in question isn't the only one effected, the error is also seen on our main .css file. The files in question are 1.29MB for the js file and 696KB for the css and we see the error the most against the js file. Rarely we've seen it against our fonts .tff file which is only 57KB and our images which are very small as they're either small logos or avatars. So the conclusion I've come to is that the size of the file increases the likely hood of the error occouring, and that it's of course only affecting static files.

Apache error logs show nothing helpful, and when reproducing the issue in a browser all you can see is the console throwing the error in question and the file loading with the end cut off at seemingly random points each time.

List of enabled apache mods:

$ ls /etc/apache2/mods-enabled/
access_compat.load  auth_basic.load  authz_core.load  autoindex.conf  deflate.load  env.load      headers.load  mpm_event.conf    negotiation.load  proxy_fcgi.load  reqtimeout.load  setenvif.load
alias.conf          authn_core.load  authz_host.load  autoindex.load  dir.conf      expires.load  mime.conf     mpm_event.load    proxy.conf        proxy_http.load  rewrite.load     status.conf
alias.load          authn_file.load  authz_user.load  deflate.conf    dir.load      filter.load   mime.load     negotiation.conf  proxy.load        reqtimeout.conf  setenvif.conf    status.load

Apache version (latest):

$ apache2 -v
Server version: Apache/2.4.48 (Ubuntu)
Server built:   2021-07-01T19:15:28