Version:           KMail 1.9.6 (enterprise 20070904.708012) (Using KDE 3.5.7 "release 72.2" openSUSE 10.3) (using KDE KDE 3.5.7)
Installed from:    SuSE RPMs
Compiler:          gcc version 4.2.1 (SUSE Linux) Using built-in specs.
Target: i586-suse-linux
Configured with: ../configure --enable-threads=posix --prefix=/usr --with-local-prefix=/usr/local --infodir=/usr/share/info --mandir=/usr/share/man --libdir=/usr/lib --libexecdir=/usr/lib --enable-languages=c,c++,objc,fortran,obj-c++,java,ada --enable-checking=release --with-gxx-include-dir=/usr/include/c++/4.2.1 --enable-ssp --disable-libssp --disable-libgcj --with-slibdir=/lib --with-system-zlib --enable-shared --enable-__cxa_atexit --enable-libstdcxx-allocator=new --disable-libstdcxx-pch --program-suffix=-4.2 --enable-version-specific-runtime-libs --without-system-libunwind --with-cpu=generic --host=i586-suse-linux
Thread model: posix
OS:                Linux

When moving to the next unread message within a folder using the '+' hotkey, kmail occasionally skips over all of the remaining unread messages in one go and marks all of the messages as read. This behavior has not been observed in previous versions of kmail.

This bug has been observed in local mbox folders containing 360 messages (50 unread, randomly distributed) and 20000 messages (15 unread). So it doesn't seem to depend on folder size. Furthermore, one of the folders threads messages, the other one doesn't. 

Usually a number of "go to next unread" commands work before kmail starts to visit all of the remaining unread messages. 

I haven't found a way to reproduce this with certainty: 

Up to now, this bug has consistently been observed if a folder contains new/unread messages as put there by kmail. 

I also ovserved it once by going to a folder containing only read messages and marking a number of messages as unread. Then I switched folders a couple of times before returning to the prepared folder. However, I just tried a few times to reproduce this and failed...