Fairly common
http://www.nasa.gov/centers/marshall.../#.Uwvrc_k0UuU
Saw one through 16" f/3.3 in 1979 though nobody would believe me at the time - the accepted wisdom was this sort of thing only happened in the distant past.
Edit:
Wrong year, it was 1981. My old notebook records that was when I swapped out the cas secondary for the newt diagonal and triplet corrector.