When passing in bad values to .set or .put, sometimes the callbacks are invoked, and sometimes they aren't. For example:

Gun().get('test').set(NaN, function () {
  console.log(arguments)
})
// the function is never called

But if you pass in Infinity instead, the callback will fire, passing in an error object with the message "Invalid value at [soul]!". But it doesn't stop there! There's even inconsistency between .set and .put (even though .set uses .put under the hood).

Gun().get('test').path('callback madness').set(Infinity, callback)
// invoked, error given. Let's try put this time...
Gun().get('test').path('callback madness').put(Infinity, callback)
// not invoked, no error! What?

These errors might be the last string of sanity that beginners have, since gun doesn't throw traditional errors.