Hola Jannice

In that part of the lesson we put two very similar sentences to highlight that the verb is used in 3rd person singular whenever you forget "doing something" ; that doing something is expressed with an infinitive. We give two examples, one showing when you forget "bringing a singular thing" (traer el hielo) and another one showing when you forget "bringing a plural thing" (traer cervezas) - even though "cervezas" is a plural object, you still need the verb in the singular form "se me olvidó" (not se me olvidaron traer cervezas). This is what we try to show with those two examples. One might think that the verb should agree with the object, but it agrees with the infinitive (in this case, traer).

I hope this clarified it. 

Saludos cordiales