What would the complications be (if any) if say when one was trying to change the rubber spout filler part of their kombi's petrol tank they accidently dropped a very small screw down into the tank???
Any major issues?

Just hypothetically speaking? Or did you DO IT?

You sound like one of those guys who says "ahm, my friend has just caught a sexually transmitted disease. What should I - ahm - he do about it?"

Love it! You "screwed" up!

Sorry, shite-stirring is my life!

Back onto the topic - Is it big enough to get lodged in your fuel line? If it's small enough, it may even go for a wander and you can collect it out of your fuel filter. It's highly unlikely to make that perilous journey tho'.

Eventually it'll corrode and have to come out. Maybe it's a nice surprise for the car's next owner? People have found all manner of things in their fuel tanks, and not had them significantly effect their ride. A little screw shouldn't be a problem.

Hmm, the Beetle has a screen in the fuel tank, can't say I looked inside the Kombi's - anyone?

The Kombi tanks also have a filter screen n stand pipe.

The only problem you will have is the extra noise of the screw rolling around inside the tank while yr driving along n making a shitty rattling sound that will drive you crazy trying to find it.