The phasing of the loop points must also be aligned, so that you align the peaks and troughs of the waveforms at the beginning of the loop with those at the end. This includes any higher order patterns that are visible when you view the sample at different zoom levels, creating different interference patterns. While misalignment of loop points usually results in a pulsating or growling effect, it could also result in a change in pitch. Making a loop a little too long or too short can have the effect of changing the frequency of the loop region, so even if the loop itself sounds normal, it may be playing at a different effective pitch than the non-looped portion of the sample. It may sound like it can be a little tedious to make a good loop, and it IS if the sample is not a good one, but if the sample meets all the criteria given above in the sampling section, it should be fairly easy to select a section near the end of the sample that will make a good loop.
Real instruments sound different depending on what octave they are being played in.
As an example, a trumpet sample of a high C will not sound like a real trumpet when that sample is stretched down two octaves. To overcome this problem, multiple samples are used in the patch, with each sample assigned to a different region of the scale. The closer in pitch the samples are to each other, the more realistic the final patch will sound. Unfortunately, using many samples that are very close to each other can produce a very large patch. A compromise must be reached between size and realism.