Glitch - Falling through blocks but not water

I was on Vans Faction Server when I found this glitch. I travelled through a tunnel and fell through blocks. Only blocks I could stand on were trap blocks and ladders, and I stood on doors but fell. All of those were entities. I also had water under me as most of the map was water and I could swim but go through blocks.

I then went around the map studying this glitch. At one point I went inside this island and I could jump 2-3 times before fallling through again. I think my singleplayer affected this as I went on sandbox and tried to run Zombie Fortress Plus (the one with assassin and crossbowknight classes etc.) but I was frozen with a red circle. I had 3 mods activated. ZFM+1.6.6a, CustomTitleScreen and Deepthroat.

I took a few screenshots of me inside dirt blocks. Here they are.

Spoiler: Glitch Screenshots