I do not believe that apt 1.2.29 with libapt-pkg5.0 1.5.1 fails with that error, btw. I believe it fails with:
symbol _ZN9pkgSystem9LockInnerEv version APTPKG_5.0 not defined in file libapt-pkg.so.5.0 with link time reference
Which is expected (since 1.2.29 gained a new frontend locking symbol), and brings us back to the point of partial upgrades not being supported.
I do not believe that apt 1.2.29 with libapt-pkg5.0 1.5.1 fails with that error, btw. I believe it fails with:
symbol _ZN9pkgSystem9L ockInnerEv version APTPKG_5.0 not defined in file libapt-pkg.so.5.0 with link time reference
Which is expected (since 1.2.29 gained a new frontend locking symbol), and brings us back to the point of partial upgrades not being supported.