$ cat e.vala && valac --fast-vapi e.vapi e.vala
public abstract class Foo : GLib.Object {
  public virtual void bar () throws GLib.Error { }
}
e.vala:2.3-2.25: error: overriding method `Foo.bar' is incompatible with base method `Foo.bar': incompatible error type `GLib.Error'.
  public virtual void bar () throws GLib.Error { }
  ^^^^^^^^^^^^^^^^^^^^^^^
Compilation failed: 1 error(s), 0 warning(s)


Doesn't happen with 0.32.  Haven't tested with 0.34.1, but there have been a few commits since then about better checking of type arguments; seems a likely place to look.

AFAIK it only occurs with fast-vapis, and only for methods which throw errors.