The note is transcribed in the description of Petscop 1 (previously an annotation in the video, before annotations were removed), and reads as follows:
I WALKED DOWNSTAIRS AND WHEN I GOT TO THE BOTTOM, INSTEAD OF PROCEEDING, I TURNED THE RIGHT AND BECAME A SHADOW MONSTER MAN
Please go to my website on the sticker and also go to roneth's room and press start and press down down down down down right start
The first part of the note may have been written by Marvin. The "Shadow Monster Man" bug is seen in use by him in Petscop 6, later replicated by Paul in Petscop 9. The "glitch" itself is performed by walking down the stairs of the Flower Shack's basement side, and walking right at the very bottom before hitting the loading trigger.