Julia's last words about a "speckled band" were in fact describing "a swamp adder, the deadliest snake in India".

The speckled band!

In the story it is mistaken as a "speckled band" many times.

Sherlock Holmes was standing alone in the whirlwind, a speckled band tied bandana-fashion across his face.

And what was 'the speckled band'?

Round his head was a strange, yellow speckled band.

The speckled band!' said Holmes very quietly.

But she never really forgot the speckled band.

"Ah, and what did you gather from this allusion to a band - a speckled band?"