I used a execute command to replace lava around you with packed ice 20 times a second.

I expected the lava to be replaced normally. but it rarely works. I know it is a bug because when i use this command but replacing water rather than lava it works.

Few pieces of lava actually turn into packed ice.

Steps to Reproduce:
1. Make a redstone block clock using fill commands with air and redstone blocks. (seen in videos by sethbling, command block on left says /fill ~-1 ~ ~ ~-1 ~ ~2 redstone_block command block on right says /fill ~1 ~ ~ ~1 ~ ~2 air)
2. have a command block connected to the clock with command /execute @p ~ ~ ~ fill ~-1 ~-1 ~-1 ~1 ~-1 ~1 minecraft:packed_ice 0 replace minecraft:lava
3. While this execute command is running through clock try to walk above lava. It will not work. Now change the command block in step 2 from minecraft:lava to minecraft:water. Try walking on water. It will let you walk and sprint on water.