My least favorite feature is block scoping combined with the := operator.

For example, this works:

  var x Foo
  x, err := doSomething()
  if err != nil {
    ...
  }

this does not:

  var x Foo
  if x, err := doSomething(); err != nil {
    ...
  }

This pattern (or some variant) introduces more bugs in my code than I would like to admit.