Board Thread:General Discussion/@comment-27907368-20160506131212/@comment-26347196-20160513155026

PancryPancreas5 wrote: It's hinted that previous six humans were able to reset. When you reset and get Toriel's phone call about pie, she remembers which flavour you choose and states that she feels like she already knew every human who fell... If you think about it, if the six souls can't reset, Toriel would retain the memories of their deaths, which is logical since she has that memory very clear even before Frisk came. When Frisk reloads, however, Toriel has the slight recollection of the pie thing.

Furthermore, if the six humans were able to reset, they are able to undo their deaths and would eventually get out of the Underground and not getting trapped in the underground.