Erik K. Antonsson, a prof at Caltech, has a page of quotations related to design and engineering. Some samples:

“If a major project is truly innovative, you cannot possibly know its exact cost and its exact schedule at the beginning. And if in fact you do know the exact cost and the exact schedule, chances are that the technology is obsolete.”
-Joseph G. Gavin, Jr., discussing the design of the lunar module that landed NASA astronauts on the moon.

“What appears at first to be well-articulated, firmly established architecture often consists of a broad (perhaps even vague) product concept; a set of evolving, sometimes loosely formulated specifications; and multiple, often conflicting targets that may be difficult to meet. The product is invariably complex and the planning process, its attention to detail notwithstanding, is unlikely to uncover all the relevant conflicts and problems in advance. To meet an objective such as `the door on the new luxury sedan should create a feeling of solidity and security when it closes’ may be difficult, involving the application of technical expertise and a great deal of negotiation with engineers working on the body, electrical system, stamping, and assembly. Though planning establishes overall direction and architecture, product engineering must still confront numerous conflicts and trade-offs in local components and subsystems.”
-Kim B. Clark and Takahiro Fujimoto, “Product Development Performance: Strategy, Organization and Management in the World Auto Industry”

“And let it be noted that there is no more delicate matter to take in hand, nor more dangerous to conduct, nor more doubtful in its success, than to set up as the leader in the introduction of changes. For he who innovates will have for his enemies all those who are well off under the existing order of things, and only lukewarm supporters in those who might be better off under the new.”
-Niccolò Machiavelli, “The Prince”