A finger fumbler is the sign language equivalent of a tongue-twister. A tongue-twister as well as finger-fumbler is a phrase that is difficult to articulate properly. It is mispronounced as a result of co-articulation through rapid alternation between similar but distinct phonemes (speech) or primes (signing).
The phrase "Good blood, bad blood" is a common example of finger-fumbler in ASL as well as tongue-twister in English. Not many examples in ASL finger-fumbler have been recorded.
A recent recorded video (2020) below shows some example of a finger-fumbler of the fingerspelled English word "collaboration." Perhaps it's not an excellent classical finger-fumbler but...
While the signer myself could fingerspell this word right very easily but it changed when fingerspelling it in the middle of a sentence. It happened twice with no further correction. This process also involved reading the text, translating English to ASL, and producing ASL at the same time. This busy cognitive process is probably a reason for the mispronunciation to happen in ASL.
Hope to see some more videos captured in the action of a finger-fumbler in ASL in the near future.