TL;DR:
when we use Vue.use to add a plugin, which render a component instance(with Vue.extend), and the component $emit events with vue instance as payload. It cause vue warns.

version:
vue(>2.0.0) vue-devtool(latest)
demo:
codepen
(can't reproduce the bug unless u copy the code, start a demo and open vue-devtool)
bug:
vue warns: Property or method "toJSON" is not defined
facts:

• JSON.stringify would try to call toJSON attr when it meets object. spec
• vue-devtool use JSON.stringify (or circular-json-es6) to deep copy(I guess) the $emit payload
• vue use Proxy(_renderProxy) to warn not defined attr or method during render in develop mode

• vue call the _init function (which included _renderProxy) in Vue and Vue.extend(produce _Ctor)

reason

• vue-devtool find out a $emit, try to use JSON.stringify, JSON.stringify meet _Ctor attr first than circular reference
• _Ctor[0] has _renderProxy, JSON.stringify try to call toJSON, cause getHandler to warn.
• it's a rare bug, because it happens only when _renderPorxy is reached before circular reference.

but, it's real confusing...