While building a castle, I noticed that while standing only partially on mud blocks and placing a silt block where you are partially floating, you will be able to walk through the silt. Once you have left the block, you of course can't get back in again without performing the glitch. At first I thought it might be because of a certain tool from the quark mod I was using to place the blocks, but after further testing without quark the problem still occurred. This only works while wearing the rubber boots or boots of the marsh runner, which is interesting to me. All testing of this glitch was done in 3.6.1 with the only non-visual/performance mod installed being initially quark, then none.