I think that there is a misunderstanding about the day that I should pick up the keys from your neighbours.

misunderstanding 是由前缀 mis-（表示“错误”）和 understanding（理解）组成的复合名词。understanding 来自动词 understand（理解），加上 -ing 变成名词形式。因此 misunderstanding 字面意思是“错误的理解”，即“误会”。