In case (iv) (E AA = E BB =0, and E AB =-2; seeded with A3B3), we considered copying to be accurate when, for example, A4B2 was stacked on top of A2B4 such that A and B are adjacent to each other. In contrast, two units of A or two units of B placed adjacent in a stack were considered errors. Inspection of the fibers (Figure and SI section S3) revealed the emergence of A4B2-A2B4-A4B2 patterns, as well as its "mirror image" A2B4-A4B2-A2B4. About 70% of all fibers contained at least one of these patterns and about 26% of the fibers exhibited this pattern over a length of six or more adjacent macrocycles in the fiber (e.g. A4B2-A2B4-A4B2-A2B4-A4B2-A2B4), while in the control without surface binding only about 55% of all fibers exhibited these patterns and only about 18% exhibit them in stretches of 6 or more adjacent macrocycles (Figure ). Surprisingly, in case (iv) the error rate for replication of A3B3 was somewhat higher than in the control (Figure ), even though all trimers bind to the A3B3 seed with identical affinity (SI section S2), meaning there is no binding selectivity. We attribute this increased error rate to stochastic effects, arising from the small number of trimers that are bound to each individual fiber: On average each fiber only has less than 10 surface-bound trimers during the early stage of growth (SI section S5), yielding stochastic deviations of their distribution from the average distribution in bulk solution. Consumption of two trimers from such a small pool to create a new hexamer may also cause such an effect. These local deviations from the bulk average trimer distribution increase the mutation rate for A3B3. Similar stochastic effects are also present for the other ring compositions, but impact replication fidelity much less, as the statistical fluctuations may also enhance fidelity, depending on whether these fluctuations bring the building block composition of the bound trimers closer to, or more distant from that of the fibers. In contrast, for the A3B3 seed all statistical fluctuations make the building block distributions in bound trimers more dissimilar from that in the fibers.