(a) observation yields limited knowledge, such that what we know is but knowledge in part. This is obvious. If our knowledge were exhaustive, we would no longer discover anything. But it is easily forgotten. If the observer problem has a problem it is that we often take what have discovered to hold for what we haven't discovered, or don't care to know, often by applying principles in one area of thought to stop the gaps elsewhere. All the -isms suffer from this, logicism, marxism, legalism, scientism.
(b) observation cannot fix the subject of study, such that "the act of observation itself disturbs the observed object, and thus changes the total system."
(c) observation results in concepts abstracted from existence, such that further theoretical speculation yields conclusions that may not hold true for the thing in the real. Thought is a world of its own. The $100 in my mind, for all its worth, is not a $100 in my pocket. A useful theory, as Ortega puts it, must "mate happily with reality" to become knowledge.
Ortega puts the problem very well indeed.
I'm not entirely sure what you intend by the word "fix" in (b). But it does seem clear to me that the observer can be seen "disturbing" and thus modifying that which he observes. Consider the case of a cultural anthropologist, for instance, who travels to a tribe of primitive people for the purpose of studying it. His very presence as a complete "outsider" of obviously different culture than their own disturbs the behavior of the people he's come to study. Or what of the claim by a literary analyst, that such-and-such book will have wide appeal among a variety of different readers, for each will find in it perspectives congenial to his own outlook.
More to cover but I must stop now: Dinner is served! And I'm hungry!!! I hope to be back later.
Thanks so much for writing, cornelis!