I'm often excited when I see students make the very errors I try to warn them off of in their interpretation assessments, because it lets me reinforce that the phonetic patterns I'm teaching are real (they often seem to only half-believe me). For example, when a student transcribes a Spanish "r" as a "t", "d" or "l", and when they transcribe an intervocalic Spanish "d" as a "v", it's a great opportunity for me to talk about what the Spanish "r" and "d" actually sound like (the "d" being transcribed as "v" is particularly interesting -- the English phoneme that matches the sound is spelled "th", of course, but students know "th" is unavailable as a Spanish phoneme so they hear another voiced fricative -- "v" -- in its place, until, that is, they've internalized the pattern that intervocalic "d"s are like our voiced thorns).
Anyway, today in my geek blog reading, I read the following in a Spanish-speaking hacker's blog:
Benchmarking software is not an easy thing to do. It may look trivial, but it requires a bast amount of time and effort when you want to do it right.
Notice the "b" for a "v" there. What that tells me is that Alvaro almost certainly learned the word "vast" orally, rather than through writing, since his spelling is otherwise impeccable, and that, in spite of being literate in English, he is still hearing English with Spanish phonemes. Which is to say, he hears the voiced labiodental fricative ("v") as a voiced bilabial fricative (β), which he understands as the intervocalic allophone of the phoneme /b/.
Of course, one possible reading of this is to say that Alvaro appears to be a fully functional English speaker who has not internalized the English phonetic system... which suggests that the typical Spanish student who does not understand that, for example, the Spanish phoneme /d/ encompasses something very like our "d" as well as an allophone that is exactly like our phoneme /ð/, which we spell "th", may not be in such a bad situation.
Nonetheless, I think it does make a difference... a student who hears the phrase "de dónde eres" as "devonderes" is surely at a disadvantage (and that error is not that atypical in my experience, provided you provide students with actual examples of Spanish speech, and not with typical American-Non-Native-Speaker-Spanish-Teacher pronunciations.