Isn't ChipKill just another form of ECC? If so there is a number of bitflips that ChipKill can no longer correct / detect. [1] seems to say that they observed some flips in dRAM with ChipKill, although the paper is a bit vague here.

[1] B. Schroeder et al, DRAM Errors in the Wild: A Large-Scale Field Study http://www.cs.toronto.edu/~bianca/papers/sigmetrics09.pdf