(i) joint (ii) Used search box for you
Very few MP3 encoders make ideal decisions about what mode to use from frame to frame in joint stereo files, so there has long been a misconception that joint stereo means too many (perhaps 100%) mid-side frames, which is rarely desirable in stereo music and can result in haphazard channel separation and/or suboptimal quality as measured by other criteria.
Simple enough: I changed "Very few MP3 encoders" to "Some early MP3 encoders didn't" and appended "This is not an issue with modern encoders." I also moved the text out of the lead and used it to replace some redundant info at the end of the article.
Some early MP3 encoders didn't make ideal decisions about what mode to use from frame to frame in joint stereo files, or how much bandwidth to allocate to encoding the side channel. This led to a widespread but mistaken belief that an abundance of M/S frames, or the use of joint stereo in general, always negatively impacts channel separation and other measures of audio quality. This is not an issue with modern encoders. Modern, optimized encoders will switch between mid-side coding or simple stereo coding as necessary, depending on the correlation between the left and right channels, and will allocate channel bandwidth appropriately to ensure the best mode is used for each frame.