The early French, for unknown reasons, decided that /S/ was fine, but not when followed by a hard consonant. In particular, they didn't like the initial combinations "sc-", "sp-", "st-", etc. they'd inherited from Latin, and systematically changed them, first adding an initial /E/ and then later dropping the /S/ entirely, so that modern French has etat for "state", ecole for "school", epice for "spice", and so on. The city of Detroit is French d'etroit, "the strait". Attach is from Old French estache, a post. Thus it is yet another "stake" word — see the long discussion about Stacy the Prostitute.
Since many English borrowings are from Old French, we also find English words from the intermediate forms — estate, esquire, espy, example, and especial stand side by side with state, squire, spy, sample, and special. Eschew is a special case because French has the anomalous esquiver, to avoid, instead of the expected "equivir"; the assumption is that they borrowed the word back from Italian or Spanish in relatively modern times. The root is Germanic skeu-, which gave English shy, shun, and skew (originally, to shy away or avoid).
Germanic skip- meant boat; it is responsible for English ship, skiff, and skipper ("shipper"). The French borrowed the word and created something like esquipe, to fit out a ship. After the /S/ got dropped as usual, this was generalized into English equip.
Most people probably assume that rotisserie is a member of the "rotate" family, from the idea of meat turning on a spit. Most people are wrong; there is no connection. "Rotisserie" is another French word where an offending /S/ got dropped — it was originally "rostisserie", a place where meat was roasted.
I've mentioned that ie /K/ is Welsh /P/, while ie /P/ was dropped entirely by all the Keltic languages. (Besides "penguin", another Welsh example is the word for "five". In most ie languages it's something like "kink', but it is Welsh "pump".) For that matter, ie /K/ mutated into /S/ in Indo-Iranian and Slavic. Linguists have nicknamed the /K/-vs-/S/ languages as the centum and satem branches, from the Latin and Sanskrit for "one hundred".
Here's another unrelated example of a single-letter-single-language change: Latin, for some reason, sometimes converted an Indo-European /D/ into /L/ instead. (Some experts think they may have acquired this quirk from the Sabines.) In any case, Germanic odor and Greek ozone have Latin cognates with ol-, such as olfactory and redolent. This is also how Homer's Odysseus became Vergil's Ulysses. Latin pilot is a derivative of the Greek pedo- "foot" words — the original meaning was a steering oar. [23Jun08] An Indo-European word for "fat" or "grease" led to both adipose and lipid.
Yet another example is Germanic tongue. As expected by Grimm's Law, this leads back to a dnghu- ie root, so the Latin version should have been something like "dingus" or "dongle". In reality, Latin has a ling- base instead, appearing in such words as language, linguist, lingo, bilingual, etc. Italian linguini are "little tongues", no relation to a lingweenie, who is a person who doesn't read dictionaries. Exactly the same /L/-for-/D/ situation is Germanic tear (liquid from the eye), ie dakr-, but Latin words in lacr- such as lacrimose, teary, and lachrymator, tear gas.
[30Jun08] To call someone a weenie, by the way, does not imply an obscure connection to a weiner sausage or the city of Vienna, although the spelling "weinie" is sometimes seen. It is really a derivative of wee — the ending is apparently from its association with tiny in the phrase teeny-weeny. Small children have been called weenies in England for a couple hundred years, and the meaning was extended to a weak or ineffective (i.e. childish) person in 1929.
An unlikely pairing is the transformation of Greek diphtheria into Latin letter. The root is an Indo-European deph- that meant to stamp. Greek diphthera meant prepared leather (vellum or parchment) used for writing, hence the Latin. The disease acquired its name because one of the main symptoms is a tough skin formed mucous membranes. Some relatives of "letter" are obvious, such as literal and illiterate, but one that isn't quite so obvious is obliterate, to erase what has been written.
[07Sep08] A good stumper of a question is to ask someone to list all the English words that contain a /phth/ (Greek /φθ/). In addition to diphtheria, the only others are diphthong, naphtha, and the ophthalmo- derivatives.
[10Jun08] As mentioned, both vellum and parchment are thin leather. Vellum is from Latin vitulus, which meant a yearling animal — veal is the same word via French. Parchment, meanwhile, is properly writing material from the city of Pergamon, in what is now Turkey.
There is even some evidence that French and Italian have pulled this trick a couple of times: Cicada was borrowed directly from Latin, but the French for grasshopper is cigale, and a further /R/-for-/L/ substitution produced cigar, from the shape of the insect. This also could explain jolly (French joli) as a descendant of Latin gaudia, joy.
Similarly, public policy has nothing to do with an insurance policy. The former is a member of the polis (city) family that also includes politics, police, etc. However, the insurance paper is from Italian polizza, receipt, and that seems to be an /L/-for-/D/ switch from Latin apodixa, originally Greek apodeixis, proof or declaration. Literally, the Greek means to show off; in apo-deik the second element is the root of the "speak" or "show" words like diction and teach, q.v.
One other quick consonant change. Spaniards pronounce (and often spell) Latin /V/ as /B/, just like the Japanese. C.f. Havana vs. Habenero. Clear back in classical Latin, the Romans had a pun that the Spanish were the most blessed people in the world, because for them "vivere bibere est", to live is to drink.