From Gerald Erichsen,
Your Guide to Spanish Language.
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Lack of "th" in Spanish: Cognates of English words with a "th" usually use a t in Spanish. Examples are tema (theme), metano (methane), ritmo (rhythm) and metodista (Methodist).

Use of inm- instead of "im-" to start words: Examples include inmadurez (immaturity), inmaterial and inmigración.

Use of es- for "s-" preceding a consonant: Native Spanish speakers have a difficult time pronouncing words beginning with various letter combinations starting with s, so the spelling is adjusted accordingly. Examples include especial, estéreo, escaldar (scald), escuela (school) and esnobismo (snobbery).

Avoidance of y as a vowel: Except for some recently imported words such as byte and sexy, Spanish usually doesn't use y as a vowel except in diphthongs, so i is used instead. Examples include hidrógeno (hydrogen), dislexia and gimnasta (gymnast).

Use of cua and cuo instead of "qua" and "quo": Examples include ecuador (equator) and cuota.

Dropping of English's silent letters: Commonly, the "h" in English words is dropped in the Spanish equivalents, as in ritmo (rhythm) and gonorrea (gonorrhea). Also, it is common in modern Spanish to not use ps- to start words. Thus sicológico is used for "psychologist," although the older forms such as psicológico are still used. (The cognate of "psalm" is always salmo.)

Use of tras- for "trans-": Many English words that begin with "trans-," but not all, have Spanish cognates that begin with tras-. Examples include trasplantar and trascender. However, there are many Spanish words where both tras- and trans- are acceptable. Thus both trasferir and transferir (transfer) are used, as are both trasfusión and transfusión.

Use of f for the English "ph": Examples include elefante and Filadelfia.

Other irregularly spelled cognates: Following are some other easy-to-misspell words that don't fit any of the above patterns. The Spanish word is in boldface followed by the English word in parentheses. Note that in a few cases the Spanish word doesn't have the same meaning, or has other meanings, than the English word listed.
abril (April)
adjetivo (adjective)
asamblea (assembly)
automóvil (automobile)
billón (billion)
carrera (career)
circunstancia (circumstance)
confort (comfort)
coraje (courage)
coronel (colonel)
diciembre (December)
énfasis (emphasis)
erradicar (eradicate)
espionaje (espionage)
etcétera (et cetera)
femenino (feminine)
garaje (garage)
jirafa (giraffe)
glaciar (glacier)
gobernar, gobierno, etc. (govern, government, etc.)
gorila (gorilla)
gravedad (gravity)
huracán (hurricane)
Irak (Iraq)
jamón (ham)
jeroglíficos (hieroglyphics)
jonrón (home run)
lenguaje (language)
mensaje (message)
millón (million)
móvil (mobile)
noviembre (November)
objeto, objetivo (object, objective)
octubre (October)
pasaje (passage)
proyecto (project)
septiembre or setiembre (September)
siniestro (sinister)
subjuntivo (subjunctive)
tamal (tamale)
trayectoria (trajectory)
vainilla (vanilla)
vagabundo (vagabond)
vasco (Basque)
yogur or yogurt (yogurt)