Here's an odd one - after a winter sitting outside under a cover (sadly I don't have the luxury of a garage) I took my scruffy old '98 BB out for the first run of the year last month, and noticed the temperature gauge reading hotter than usual. I checked the rad was hot (it was) and the coolant level was OK, so I put it down to getting stuck in traffic.
More recently I took it out again and suddenly the gauge needle went almost vertical off the scale I rode it home gently, but the engine didn't seem any hotter than usual and no coolant was leaking out so I suspected the gauge was lying.
After a couple of hours of head scratching I eventually traced the problem to the temperature gauge connections at the back of the instrument cluster. Corrosion between the gauge fixing screws and the copper strips was causing a higher resistance and fooling the gauge. Cleaning the screws and washers with contact cleaner and a glass fibre brush was all that was needed, and now the gauge is back to normal. Hopefully a dose of anti-corrosion spray will stop it recurring next year.