The noun supellex takes the stem supellectil- in declined forms. The -il- part seems related to the suffix -ilis as in fragilis, but it disappears in the nom. sg. form.
(By the way, the expected nom. sg. form supellectilis is also attested.)
According to Wiktionary, it comes from an unattested verb *superlegere. It reminds me of another noun supplex, which comes from the verb supplicare but without -il- in its declined forms.
How did this happen? Are there any similar examples for abnormal declension patterns?