While putting in support for deserialization of delta CASes which update existing FS which may be indexed (thus needing special handling to avoid potentially corrupting the index), I noticed what appeared to be a kind of copy-paste error - where a wrong FS feature offset variable was being used.
This would only ever show up if form 6 was being used and the source and target type systems differed in a manner that led to different offsets in the FS for the affected kinds of features. The affected kinds are: byte, boolean, float, and string, and arrays of float and string. The typo was that the heap update was using the cas-being-deserialized'd feature offset, instead of the cas-being-deserialized-into's offset. Other kinds were correct.