Double a final consonant before a vowel suffix when the word ends consonant-vowel-consonant and is stressed at the end.

The doubled letter is what keeps the preceding vowel short. Without it 'refering' would be read to rhyme with 'peering', which is why the rule exists at all.

Refer becomes referring; prefer becomes preferred.

stress on the last syllable, so doubled

Refer becomes refering; prefer becomes prefered.

the doubling omitted

Where it bites

Words where the stress moves. 'Refer' is stressed on the second syllable and doubles; 'reference' moves the stress to the first and does not.