You can sometimes get baby Mario stuck so that Yoshi can't reach and save him. One of these areas is in Bowser's castle. Once you go past Kamek, try to gain access to door 4, which has many coins and the ground moving up and down. Go on one of these platforms, preferably one that is a peak. Get hit by a ghost. Now, go on the other side of the area of the ground and lick baby Mario over or hit him with eggs. You want to get him sort of positioned on the peak (where Yoshi is standing in the second picture) when it's down, so when it moves up, his bubble will go through the ground. He will become stuck under it and you won't be able to reach him. You just have to wait for Kamek's people to come and take baby Mario away... how fun. You can actually get Mario back if you're nimble. You have to wait for Kamek's people to grab him and take him through the ground and then take him back.
Submitted by: kornyhatemachine