Some reactive languages are glitch-free, and prove this property. This is usually achieved by topologically sorting expressions and updating values in topological order. This can, however, have performance implications, such as delaying the delivery of values (due to the order of propagation). In some cases, therefore, reactive languages permit glitches, and developers must be aware of the possibility that values may temporarily fail to correspond to the program source, and that some expressions may evaluate multiple times (for instance, codice_8 may evaluate twice: once when the new value of codice_1 arrives, and once more when codice_2 updates).