A typical example is when a node contains a question, and a link to the answer. If the node containing the answer doesn't (at least partially) repeat the question, this creates a cognitive overhead for the reader who must remember the question while reading the answer.
Link anchors that appear within the text (like with this course text) may motivate the user to click on the link before finishing reading the entire current node. Jumping from node to node, reading only part of each node each time, generates extra cognitive overhead.
Adaptive hypertext tries to eliminate some of the cognitive overhead by providing extra context when appropriate, and by changing the link structure so that inappropriate links are never followed inadvertedly.