You may not know what it is by name, but you've probably done it yourself lots of times, and if you've been around kids, I'm sure you've heard more examples as they learn to talk.
Metathesis is when two sounds get flipped in the pronunciation of a word.
For young children, the most familiar one might be saying pa-sghetti instead of spaghetti, where the child exchanges the places of the S and the P.
A common example among adults is how they say the full word we write as etc. Many people say ex cetera.
That's not technically correct, but a lot of us say it. The term is really et cetera, with a T, not an X. That pronunciation makes a lot more sense when you look back at the shortened version we all use, etc.
When my son was little (and because I'm a total word nerd), I loved watching his language develop. A version of metathesis he came up with was with "granola bar."
He dropped the R altogether, as young kids often do, since it's a hard consonant to say when you're two years old. Then he exchanged place with the N and the G, but kept their accompanying vowels. So GA was swapped with NO. Oh, and two syllables was plenty for this kid.
The result: GRA-NO-LA > (drop the R and last syllable) > GA NO > (add metathesis) > NO GA
He often asked for a "noga" bar.
Below is a short video explaining more about metathesis and how this linguist had students catch her on a word she often says "wrong." She also discusses some unexpected English words that have changed permanently over time through metathesis.