No-one is obligated beyond what he is able to do is a corner stone of the common law. However it also obligates that when you are offering services beyond your ability you are obliged to point out this missing ability. In the building architecture in most countries it is a legal requirement to point out what aspects of any job falls in core competency, what is covered via non competencies and the areas that require an additional external source.
For us as architects it should be similar, with knowledge lacks clearly published. Yes there is learning on the job, but while you learn you are not a master and the kind of practices in the early days of architecture need to be replaced by professional rules. It is really sad that when you talk with many architects they have an inbuilt panic to admit lack in understanding. So we need to ensure that we point out areas where we just have no grounding or where we have forgotten virtually everything as we have had no practice. Only when we can admit ignorance we will be seen as true professionals.