Radioactive decay is constant.
Actually, while decay is constant, the amount of atmospheric radiation has fluctuated. Because of this, some radiocarbon dates can be off if we just used decay rates. If I were a creationist, I'd probably bring this up to argue that God changes the rules.
Of course, it could still be easily refuted because science isn't a one-trick pony. Everything in radiocarbon has been calibrated first by tree rings and then glacier layers. It's really quite consistent and makes Young Earth theories highly implausible.