Folk etymology is when a word or phrase is changed – phonetically, orthographically, or both – to better fit a mistaken idea about its origin. It’s why some folk call a hiccup a hiccough: hic-cough may seem more plausible or comprehensible. The original impulse, says Arnold Zwicky, is “to find meaningful parts in otherwise unparsable expressions”.
So asparagus is sometimes written as sparrow grass, much as chaise longue → chaise lounge and coleslaw → coldslaw (which also count as eggcorns – sort of distributively limited folk etymologies). Many remain incorrect or restricted to small groups, but some become standard: penthouse came from pentis, lapwing from lappewinke, and hangnail is a modified Old English agnail.
Most people would probably assume that shamefaced comes from “shame-faced”, but the word was once shamefast, literally “restrained by shame” (fast as in “held firm”). The idea of shame manifesting in a person’s face motivated and sustained the alteration.