Playing version 2007-07-10:
1) Had to skip level 23 (Mind over Matter) because it's too hard for me. Good that this skip feature is there, because any level can be too hard for somebody.
2) When the numerical manipulation brains appeared (first the one with the "+" sign) I incorrectly assumed that they would take any input numbers and output the sum of them (ignoring non-numerical input). That was what the "+" sign signaled to me. It took me several trials to find out that they are increment brains (thay are only called that in a later level). I suggest changing the images to read "+1" and "-1", resp. and calling them increment brains on the very first level they come up.