I've been told that the first syllable of abiciō is long by position, because it's actually an underlying *abjiciō, which causes it to be syllabified as *ab-ji-ci-ō before the *ji simplifies to i. So the first syllable has a coda consonant, despite the next syllable being headless.
This makes some sense, and explains Catullus XXV.9:
ho̱c tu̱ qua̱mlu̯be̯t a̱bi̯c' e̱le̯va̱que̯
You can ignore this and laugh it off as much as you like
(The first syllable of abice here has to be long to fit the meter.)
But I've never heard of these "invisible yods" before. Are they a real thing? And if so, are there other notable ones I should be aware of, apart from the compounds of jaciō?