Das spottische auflauern ob nicht ein umstand der wirklichkeit undersporit—which is yet never attainable—and if attainable would disappoint the very purposes and ends of the Drama, demonstrates not good sense, but an utter want of all imagination, a deadness to that necessary pleasure of being innocently—shall I say deluded? No! but drawn away from ourselves into the music of noblest thoughts in harmonising sounds—Mem the passage—Blest he who not only in the public Theatre—etc./Foakes does his best with this: since undersporit is not a word in German, or in any language, he consulted his German-speaking friend René Wellek, who suggested the emendation ‘widerspricht’, which he adopts. His footnote reads: 'EHC's scrawl here and the question mark he added suggest he had no idea what he was copying.' Ouch. Foakes adds: 'This paragraph relates to C's theories about imitation (as distinct from copy) and dramatic illusion: see Lecture 4 of the 1808 series.' He also translates the German for us: 'The scornful lying in wait to see whether any circumstance conflicts with reality' [CC 5:1, 528]. He doesn't track down the source of the German, and rather implies that it might be Coleridge musing aloud (as it were) in Deutsch.
Actually, though, the mangled German is from Schlegel's Ueber dramatische Kunst und Litteratur (it should read: ‘Das spöttische Auflauern hingegen, ob nicht irgend ein Umstand der scheinbaren Wirklichkeit widerspricht’), which we know Coleridge didn't read until the very end of 1811, so the reference back to 1808 is a touch misleading. Also, when placed in its context, we can see that the whole of this Coleridgean paragraph is him summarising Schlegel's point, not making one of his own:
Die wahre Täuschung besteht eben darin, wenn man durch die Eindrücke der Dicht und Schauspielkunst so hingerissen wird, daß man die Nebensachen übersieht, und die ganze übrige Gegenwart vergißt. Das spöttische Auflauern hingegen, ob nicht irgend ein Umstand der scheinbaren Wirklichkeit widerspricht, die, strenge genommen, doch niemals vollkommen zu erreichen steht, beweist die Ohnmacht der Einbildungskraft und die Unfähig keit getäuscht zu werden. Dieser prosaische Unglaube kann so weit gehen, daß es den theatralischen Künstlern, die unter jeder Verfassung der Szene gewisse Vergünstigungen bedürfen, ganz unmöglich fällt, durch ihre Hervorbringungen die Zuschauer zu ergötzen, und so sind diese am Ende die Feinde ihres eignen Genusses. [Schlegel, Ueber dramatische Kunst und Litteratur (2 vols; Heidelberg 1817) 2:259]Here's how John Black translates that passage, in his 1815 translation of the German's lectures. After a bit of Schlegel on scene-painting ('The poet was not obliged to consult the scene-painter to know what could or what could not be represented; nor to calculate whether the store of decorations on hand were sufficient, or new ones would be requisite. He imposed no constraint on the action with respect to change of times and places, but represented it entirely as it would have naturally taken place: he left to the imagination This call on the fancy to supply the deficiencies supposes, indeed, not merely benevolent, but also intelligent spectators in a poetical tone of mind') we get:
That is the true illusion, when the spectators are so completely carried away by the impressions of the poetry and the acting, that they overlook the secondary matters, and forget the whole of the remaining objects around them. The lying censoriously on the watch to discover whether any circumstance may not violate an apparent reality which, strictly speaking, never can be attained, is a proof of inertness of imagination and an incapacity to be deceived. This prosaical incredulity may be carried so far as to render it utterly impossible for the theatrical artists, who in every constitution of the theatre require many indulgencies, to amuse the spectators by their productions; and in this manner they are, in the end, the enemies of their own enjoyment. [John Black’s transl. A Course of Lectures on Dramatic Art and Literature, by August Wilhelm Schlegel's (1815), 2:270-71]