Mutation or mistake?

Nah, nothing to worry about, your plant has "personality" and the funny leaf is just something that happens from time to time and for no apparent reason. If you grow cannabis long enough, you will see all sorts of strange leaf colours and shapes too, they are not necessarily "problems" or any sort of deficiency or excess, just funny leaves, plain and simple. They will certainly need transplanting soon though, not much room for root growth in those cups and at this stage they need plenty of room to grow their best, this is after all their foundation time and keeping them in small cups will restrict their growth at this most vital and important stage of their development. Hope this helps,......... Organoman.