The following all express the need to understand the limitations of engineering calculations

• "Don't do any calculations unless you already know the answer" (Bruce Wood)
• "Engineers need to be continually reminded that nearly all engineering failures result from faulty judgments rather than from faulty calculations" Eugene Ferguson (1992) Engineering and the Mind's Eye, MIT Press.
• "For a successful technology, reality must take precedence over public relations, for Nature cannot be fooled" - Richard Feynman (1988) from the Commission Report into the crash of the shuttle Challenger.
• "A machine or component will behave as it must, not necessarily how you calculated it would" (David Radcliffe)
• "Design calculations are only as good as the assumptions on which they are based" (David Radcliffe)