On Sun, 25 Nov 2012 17:18:44 +0000 (UTC), Steve Firth
wrote:

Which type of bulb can be fitted upside down if it matters? None I've seen.

Just about all of them if one is hamfisted enough. My local garage reckons
its one of the more significant causes of MoT failure.

Not so much upside down, but on some H4 lampholders it's possible to
work only by feel and think it's in, but it's not seated correctly.
I'm not without a clue and yet managed to do that last winter or two
ago. I didn't realise until the next night I used the headlights in
earnest and the nearside wasn't showing a proper beam at all.