Planks being displaced when going in Edit-Mode

Hello Mr. Trammell Sir and community,

Could you please have a look at this issue? I am not sure if it is an error in between the keyboard and the chair or/and bug. I have followed your instructions in the tutorial to the 'T'. Everything looks fine in Object-Mode but when I switch to Edit-Mode the planks in the back mysteriously get displaced. Now when I select the board under the first one from the top a different pair of boards get displaced in time and space... What did I do wrong? (Please see Pictures blow)

Please also find the file here, if needed: FILE (~160 KB)