That's an interesting question, and there would certainly be trouble if the system were doing crosses panmictically (between any two classifiers). However, in XCSI, the GA occurs only in the action set, where you can be sure that every classifier matches the input. If that's the case, the example you give could not occur, since there is no input that would match both 1122 and 1111. I believe that the conclusion is correct that within the action set you cannot produce an invalid classifier by crossover, but it may be worth thinking about some more...

Crossover: After crossover, there is the possibility that one or both of the offsprings' condition is not valid. I mean, l > u. For example, if I have 1122 and 1111, and the crossover point is position 3. Then after crossover, I get 1121 and 1112. So the first one is not valid but the second is. so my question is what to do if this situation happens.