Customer doesn't know why they're getting H27 errors on their app.
The causes of H27 errors are often hard to pin down as it involves the client (i.e. the users browser) opening a request and then closing the connection before a response can be returned.
We see this most often when apps have a user base with a high number of mobile clients in areas of poor connectivity. It can also happen on lossy public wifi or other problematic internet connections. As the error occurs at the client level it's impossible to identify the root cause.
You might think this an indication that your code could be optimised and made to run faster, reducing your chances that users abandon the site. However, this is very unlikely the issue. H27s can be a bit frustrating to solve since the cause is from the client and not the servers, but it's not considered a critical error.
This doesn't indicate anything wrong with your Heroku app or the Heroku router. The client initiates the disconnect from the router -- not the other way around. The H27 error is simply a report of the fact that this event has occurred and does not point to a Heroku Platform issue.