This past Sunday I bottled a BlackberryAle which was a total experiment. I didn't follaw any recipe, I just put together a very simple beer ,and added some fresh Blackberry puree into the secondary. It had a nice steady secondary fermentation. I racked it to a tertiary where it sat for about 7 days. I always try to be as sanitary as possible when racking, I use StarSan. When I opened the lid to the tertiary, I thought it smelled a little sour, not horrid but slightly sour. I also noticed a very slight film on top of the beer, almost like an oil slick. This "film" was not across the entire top of the beer and could only be seen when looking at the right angle while the light was reflecting it. The beer tasted okay (obviously green, and it finished drier than I hoped).
So, is the "film" an infection, or does fruit lend something to the beer that would be similar to what I describe?
I have never had an infection to date, so I really don't know what to look for.