Nick Heidfeld ended Nurburgring practice perplexed after failing a run spoilt by traffic and damp conditions proved to be his best lap.
The German was 11th quickest at the end of the day, but did not improve his time as the track dried after a shower - to the German's surprise.
"For me the outcome of the second session is somewhat difficult to understand," said Heidfeld.
"After the track dried out I was quite pleased with the performance. I had traffic on what eventually turned out to be my fastest lap and lost about four tenth behind a Ferrari.
"Because of this I had high expectations for the final run on another set of new tyres and on an improved track. But then I wasn't quicker than on my blocked lap earlier in the session. This is really strange and has to be analysed."
His team-mate Robert Kubica was three places further back and pessimistic about BMW's prospects this weekend.
"This was a very difficult Friday," he said. "The weather conditions were far from good. We had some rain in both sessions and it was very cold all day long.
"The weather makes it hard to keep up the tyre temperatures. It was especially difficult to get the harder tyre compound to work. We have to improve the car's balance for tomorrow. We will now analyse the data and see what we have to change for tomorrow."