The issue was that errors caused by illegal candidates (according to GHC stage or being internal names) were leaking to the user, causing bewildering error messages. If a candidate causes the type checker to error, it is not a valid hole fit, and should be discarded.
As mentioned in Trac #15321, this can cause a pattern of omissions, which might be hard to discover. A better approach would be to gather the error messages, and ask users to report them as GHC bugs. This will be implemented in a subsequent change.