Had a little flight in Sweden this weekend relating to this thread.
Was out cruising around with a friend in the club's Warrior and picked up a lot of bugs on the windscreen, not so that it disturbed visibility, but a lot of bugs, like 50-60 on the left half of the screen. We went up higher to 2500' and cruised around for 1/2 an hour when it started to drizzle very light rain. The drizzle I'm talking about here was more like moisture forming in tiny drops on the screen, not rain, and this started to dissolve the bugs resulting in an incredible messy sludge very much imparing visibility. The drizzle was not enough to remove the bugs so it only got worse. I turned back toward our arifield, which was about 20nm away. Fortunately the drizzle started to form bigger drops which started to wash the screen clean, and when i entered the circuit visibility was good enough to land.
Not a thing that I would have thought before hand could be a problem, and had the drizzle stopped, the "sludge" dried and had the sun broken trought the clouds this could certainly have lead to at least a complicated landing with very bad visibility trough the screen.
Another day, another experience to learn from, maybe they should include "bug density" as a SIGWX in weather NOTAMS ;-)