tar drops files on errors (silently)?

[Parity Error]

I tried to tar a dir from a cd (this has some difficulty in reading)
to a hard disk. (For some of the files(like 4 out of 6000files) it
reports errors, but the files are there in the tar file (would they be
of size 0 or how much ever was read or .. ?)), At the end tar finishes
and says "Error exit delayed from previous errors".

When i untar the file i find that more than 100 files are missing <--
these files where never reported by tar at all. After this i found out
that an entire dir was missed, so i cd'd into that dir and did a tar
-cvf - . | tar -tvf - | wc -l, and all the files appear to be getting
properly tarred.. But why did it miss them the first time ??