Adult learning principles, as presented by Malcolm Knowles are very commonly used by trainers as the bedrock of their activities. We'll be critiquing these elsewhere, but one major problem is that they describe adult learners in an idealized way, rather than actually describing adult learners as they actually are.
One of these principles states that adults are self-directed learners, which is true for some in some situations, and is not true for others in other situations. Many adult learners, for example, due to their learning and exposure to schools, or just naturally, do not take initiative for their learning, do not want to control what they are learning all the time, and occasionally want to allow those with more expertise to design their learning (which makes considerable sense).
When a trainer believes all adults are self-directed learners all the time, the consequences can be negative for both the trainer and the participant, and for learning. For example, let's say a trainer decides to have learners teach each other by preparing presentations on, let's say, adult learning. The assumption would be that each learner will a) choose a relevant and useful topic, b) do a good job out of pride and self-direct to do so and c) have the skills necessary to carry this out.
Those three assumptions almost never hold, and the result is poor presentations, occasional sloppy work, and the ignorant teaching the ignorant, not to mention that some learners will be so lacking in this universal self-direction, that they will have to be extensively coached and hand-held.
Yes, adults can be self-directed learners, but so can children. Assuming that all adults will be self-directed all the time results in the risk of shock and awkwardness on the part of the trainer holding that assumption.