Spoonerisms (so named for Reverend William Archibald Spooner, 1844–1930, who exemplified the quirk) occur when one transposes the initial sounds of a group of words, sometimes forming new ones, (e.g., sly gap --> guy slap.) My favorites are the ones that add new meaning or commentary to the original.
The thing is, I'm doing this constantly. As I'm shopping, I reach for a packet of bound grief at the meat display. When Deborah requests that I be on dish duty, I ponder whether I should wash dishes or dash wishes. Some of them even make it out of my head, like telling a giggling Fiona that we should have named her "Sue Tilley."
Over the years, I've been mentally gathering material for a story where one of the main characters talks in nothing but Spoonerisms, and, at the end, says something completely normal, which, no one realizes until it's almost too late, was another Spoonerism, setting the scene for the final climactic actiony bit. (The setting is, of course, a greasy spoon...er.)
So, that's one quirky thing that's always going on in my head. What's going on in yours?