When reading about other languages that have unaspirated voiceless stops, a common comparison is with the pronunciation, in English, of stops after word-initial /s/, as in "ski" vs "key", "stop" vs "top", and "spool" vs "pool". Supposedly, while /p t k/ are aspirated for most native English (and German, for that matter) speakers in initial position, they're unaspirated (but still voiceless) in English after /s/, which is the pronunciation to use in languages such as French or Spanish. (And some languages, such as Hindi, even differentiate between aspirated and unaspirated voiceless stops.)
Now, I never heard that in my own speech; I suppose I was influenced by the fact that phonemically, they're the same sound: /p t k/, so they feel the same.
I just got another bit of feedback to underline the validity of that, though :)
At breakfast, I told Jana that she had a spoon, and she repeated it as /by:n/ (as if spelled "Bühn" in German)—and I'm guessing that she interpreted the unaspirated voiceless stop as equivalent to her unaspirated voiced stop /b/ (same aspiration) rather than as equivalent to her aspirated voiceless stop /p/ (same voicing).
As to the pronunciation of the vowel, this is due to the realisation of my English /u/, which is not cardinal [u] but (at least sometimes)... something else which I'm not sure how to describe. I'm not sure whether it's less rounding, or whether it's fronting, or something. I'm guessing that she felt it was closer to her /y/ than to her /u/, at any rate.