A number of box sets and sampler discs made available prior to the April 11 release date (a result of an earlier UK release date as well as pre-orders shipping early and advance copies) have the incorrect mono mixes on Beatles VI and Rubber Soul. While the original mono mixes were intended, these sets only have mono fold-downs of the stereo versions. The mistake was understandable, as the US mono mixes of the vinyl release of The Early Beatles and Help! were fold-downs of the stereo mixes. But, the US vinyl release of both Beatles VI and Rubber Soul actually had different mixes. It was initially unknown whether this error was restricted to one pressing plant, or all pressings, but now seems to involve all sets prior to the issue of the "corrected" version.
Beatles author Bruce Spizer, who also wrote the set's in-depth liner notes, told The Beatles fan website whatgoeson.com that a "third party mastering facility incorrectly sent stereo-to-mono mixdowns" to be pressed rather than the vintage mono mixes.
The "folded down stereo mixes" are actually the stereo mix consolidated into both speakers. Although the mixes are technically in mono (in the sense that the sound in both the right and left channels is the same), they are not the actual mono mixes released by Capitol in 1965. There are some very noticeable differences, such as the actual mono mix of "I'm Looking Through You" (on Rubber Soul) lacking the false start found on the stereo mix, while the fold-down mono includes the false start. The differences between the stereo and mono mixes are one of the main draws for collectors.