The new, default Safari 5.1 behavior for finding text on a webpage (keyboard shortcut command+f) is “Starts With”, and not “Contains” like it was in older versions.
This means that if you’re looking for the word “Expression”, you’d have to begin by typing “Exp”.
Entering a portion of the word by starting somewhere in the middle, for instance “xp” or “ess”, would simply give you a “Not found” result.
To revert to the old Safari behavior, simply start a Find operation, then click on the magnifying glass and select the “Contains” option. Safari remembers your choice until the next time you manually change it again.
If you feel like using a more technical method to force “Contains” to be the default behavior, follow this plist hacking tutorial by brunerd.