NOTICE: Q/A transfer from Vocaloid wiki's Mythbusters page
Croakiness - It is a sample handling related problem and results in a sharpeness within singing results at the start of some words. The cause is transaction failure when the engine doesn't blend diphonetics particularly well. Big Al will often have this, although he is not the only Vocaloid with it his counterpart Sweet Ann doesn't not croak her words as noticeably. It is easily removed by adding echoes or playing with the pitchbend.
Choppiness - When there is missing sounds or noticeable differences between each sample in the voicebank. Luka's English suffers from this due to lack of some English diphonetics. The Kagamine's have also been reported to suffer from this in Act 2 due to some variations across the samples, though Act 1 was much smoother.
Slurs - This is caused by the engine failing to fit the samples within the given note space and instead trips over the sounds. This can potentially effect clarity though this is not always the case. Breaking up the phonetics often resolved this problem.
Silent notes - Changing the attack, transferring to another track are methods of resolving this issue. The cause behind the note is a lack of data, although with English phonetics anything less then half a note may cause a problem if unedited due to the lack of room for samples to sound out. In the case of the Kagamine's Act 2, they do not have a phonetic within their data banks so this particular sound does not exist in their capabilities at all.