The first one we all do, and that's switching pesos and som in our heads. I do okay when I'm shopping, because I have to pay attention constanstly to everything while shopping, but other times I realize I'm thinking of som instead of pesos, and since there's a big difference between the two, it really throws me off. 200 pesos is about $16 while 200 som is a little less than $5 (unless the exchange rate has changed dramatically). This works in our favor sometimes (like when oldest son clued in he'd been paid $12 for baby sitting instead of $4), and sometimes not, like when I realize that book I though was so cheap was actually not. Sometimes I end up going from som to pesos since som are automatic for me and it's an easy if not efficient 3:1 conversion.
The other thing is y and и. Russian and Spanish are kind enough to have the same word for "and" that's pronounced "ee," although they're written with different letters, obviously. But "y" in Russian is pronounced "oo" and is an annoying preposition for an English speaker, as prepositions often are. So if someone is speaking to me, I don't notice a difference, but if I see "y" in Spanish, I don't automatically think "and." Instead, if I'm not careful, I think "preposition that depends on the context." That led to the silly mistake of thinking that a store was closed from Christmas to New Years instead Christmas and New Years. У doesn't even mean until or through, but it definitely doesn't mean and, so it didn't translate that way in my head.