The second movement of Mozart's String Quartet in C «Dissonance», K. 465 has an interesting two bars where it appears that the violin theme is accidentally missing.
As highlighted in red on the score, the violin part is devoid of the crotchet/four quavers that the other parts present - the other "missing" bar is at this section's mirror in the subdominant, bar 75.
Some printed editions have added the missing notes, claiming that Mozart made a mistake (or overruling him on musical grounds), but it seems bizarre that Mozart—who even likened the composition of the "Haydn" quartets to a long, labourious fatigue ("lungha e laboriosa fatica")—would make such an error. The preface to the Neue Mozart Ausgabe agrees:
It is unthinkable that Mozart [could] have made a mistake or from absent-mindedness made a slip of the pen no less than four times in such an obvious and important detail.
(The preparation of the original first edition, which Mozart himself presided over, presumably accounts for the other two.)
At least one scholar has claimed that the notes aren't missing at all; they are simply contained within the cello's notes. I don't find this explanation particulary satisfying.
The NMA goes further by stressing its wish that the original version should be used in performances, which seems to suggest that adding the notes is (or at least, was) a common practice, but I cannot find many recordings with the added notes: my Leipziger, Emerson, Alban Berg, Ysaÿe, Italiano and Amadeus Quartet interpretations all omit the notes, whilst only the Hagen Quartet add them.