Ha Ha! We have been having this too. We started cereals about 3 weeks ago, and her diapers are now starting to change slightly from that...but I do know what you mean...every once in a while even before the solids I'd get some crazy diapers now and then. I just chalked it up to growing up, body system changing, and eliminating what her little body didn't need at that moment.
Just last night, my husband changed a crazy diaper that he described as "foamy...almost carbonated". Too funny.
has your child always been like this or just recently? DH's cousin had some sort of digestive problem as a young child -- later outgrew it, but his mom talked about horrible nasty smelling foamy green stools. Had all kinds of tests and finally she gave him rice cereal with banana and that seemed to sort of stabilize him. Of course in this situation, the child also had failure to thrive (wasn't gaining any weight) and they were concerned it was celiacs or CF.