Artistic License – History: An elderly couple tries to buy the Ingalls farm because they lived on it 40 years earlier (roughly 1836 since other episodes in that season are set in 1876). This is extremely unlikely in real life since Minnesota had very few settlers as late as 1848, and Walnut Grove itself wasn't established until 1870 (though in the series, it was founded in 1840).
Despair Event Horizon: The loss of their crop after everything seemed to indicate that they'd have a vast one for once shatters Charles to the point of giving up and making plans to go back home in Big Woods.
When Mr Simms talks to Laura about how she prayed that her father wouldn't give up on the farm, he points out that deep down she was only thinking about what she wanted, and that even though God doesn't mind people asking things for themselves, He doesn't approve of people being falsely selfless.
Charles himself realizes, on the eve of their departure, that he was ignoring his wife and daughters' reluctance to move from Walnut Grove and only thinking about his own feelings after losing the crop. He decides they should stay regardless of no longer having a house, but thankfully, Matthew Simms has second thoughts of his own and realizes the Ingalls family now belong in Plum Creek, so he gives up on buying their house and the Ingalls family continues to live in Walnut Grove.
