If you don't want to know the circumstances surrounding Tuesday's entry in this slideshow, then don't read any further...
[spoiler]Tuesday's quote:
Ep #694 (1969) - Prof. Stokes - 'We MUST examine every possibility very closely as soon as we make sure Amy Jennings is all right.'
From "Robservations 10/18/02 - #694/695 - Roger Becomes a Believer"
Quentin once again takes over David's mind. By the time the adults arrive downstairs, Quentin is gone and David is racing for the outside doors. Roger pursues his son. Liz asks Stokes why David ran from the house, assuming her nephew must have seen HIM. Roger would insist there's no one there to see, predicts Stokes. Liz wonders why the music has ceased. Our friend has no further need for it, says Stokes--we have to examine all possibilities, but first check to make sure Amy is all right.
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This quote is a rare instance where the show uses "must" (as in "must examine") but the Robservations uses "have to" (as in "have to examine") - all too often it's the other way around...
On the other hand, Liz does indeed use "must" when speculating why David ran from the house...