This has to be, by far, my favorite type of speech error (having a favorite speech error is certain to qualify as a good nerdicator, as well). This error is marked most commonly by a sort of metathesis or change in the first consonant (or onsent) of two separate, but adjacent words. Take a quote attributed to Spooner and its intended utterance:
"The Lord is a shoving leopard." vs. "The Lord is a loving shepherd."
sh l l sh
You can see that "sh" is replaced by "l" and vice versa. Part of why I love this speech error is that it acts as an example that substantiates the existence of syllable structure. Having trouble getting to sleep? I encourage you to read an academic paper on spoonerisms and syllable structure. There is enough written on these topics to give you good sleeping material for over a year.