Bad layout of articulations and slurs
In the attached score, the contents of measure 2 are exactly the same as measure 1, except that in measure 1 first the downbow was placed over the first note, and then tenutos were added, whereas in measure 2 (the problem measure) the tenutos were added and then the downbow. (It could be any articulation in place of the downbow.)
One way messes up the slur, the other doesn't.
Interesting to note: deleting the downbow in the first measure and then undoing the operation makes it the same as the second.