Maybe they thought they contained blood:http://www.balashon.com/2010/04/agvania.html
Alternatively, perhaps someone was told that raspberry flavored products were not kosher (presuming that they had carmine added for extra color). The listener understood this to mean that raspberries were not kosher.But I like Balashon's answer. It reminds me of a story I heard as a kid about someone who visited Rambam to check that he was fully Orthodox (!). The visitor saw some troubling things such as a servant named Peter and an instruction to "kill" a calf rather than shecht it and so forth. But the ultimate one was when the visitor was served a vegetable that looked like human fingers. It is needless to say that there was a reason behind each of these things - the servant named Peter was named after a Ba'al Tosfot named Peter (or Pater); the calf was one delivered from a properly-shechted mother, and the vegetables were carrots, I think. But it's a nice indication that people once had a less-than-clear understanding of the difference between vegetable and animal life.
Rasberries, or Raspberries?Phil(No need to post this typo-correction)
Maybe he meant giving a Rav the raspberry.