I noticed this bug when I worked on #1332 I accidentally returned an array of types from typeResolver function and got this error:

Abstract type FooInterface must resolve to an Object type at runtime for field
Query.foo with value \"[object Object]\", received \"FooObject\". Either the 
FooInterface type should provide a \"resolveType\" function or each possible types 
should provide an \"isTypeOf\" function.

The relevant part here is:

must resolve to an Object type at runtime ..., received \"FooObject\".

It required me some time in the debugger to figure out that I wrapped GraphQLObjectType into an array. But it's impossible to figure out this from error message itself:

Since calling validate and validateSchema are optional almost any error can be affected by this problem. So I think we need a library-wide solution to this problems.
The ideal solution would be to use utils.inspect instead of String but it is Node-specific and couldn't be used in browsers.

That's why I created jsutils/inspect.js:

export default function inspect(value: mixed): string {
  if (Array.isArray(value)) {
    return '[' + String(value) + ']';
  }
  return String(value);
}