Weird Terraria Glitch

I was playing terraria, and I went down a hole i built with a 1 block opening at the top which i use my hook to fit into. The hole was 2 blocks wide and goes down until level 15 underground. at the bottom there is a 1 block deep 1 block wide hole on the left. As soon as I got in the hole I summoned my slime mount as I fell. When I reached the bottom, instead of landing, my map replaced the bottom of the hole with a long white bar and i started falling through the ground. I stopped when I fell into a nearby cave. Can someone please explain?:(

Weird Terraria Glitch