Back to evil things PCs did. I once stole 4 cars in one run. Not to fence them; My van was in the shop so I needed an expendable transport vehicle, but I kept losing them. As I recall, one of them crashed, one of them got shot up, one of them had to be abandoned because the cops were looking for it, and the last one got light damage from running over an enemy, but lasted until the end of the run, when I had to send it screaming into the river on autonav with a cadaver in the driver's seat to cover my tracks.

The suckiest thing my players ever did, and still do, is know the rules better than I do. We trade off GMing, and even though I've been playing for eight years, I don't obsess about the books as much as some others. This is less of a problem for me now as it was when I first started GMing. The example:

Setup: Large, powerful party hired by Aztechnology (which they didn't figure out for quite a while) to go to a secret vault in the Vaticin and steal one of the Necronomicon books (I was young). It's a hugely powerful book of corrupting evil, but anyway...

Characters: Various and assorted including an assassin physad with an Ereil Flinn schtick (swooping in on ropes), and a kangaroo shapeshifter mage who was blatantly breaking rules because the player knew the rules better than any of us (he was teleporting).

Basically, after many adventures and hardships, they get the damned book, figure out what it is and who hired them.

Players: Aw, crap.
GM: Heh.
Players: I guess we'll have to arrange a meeting place.
GM: Yep.
Players: 'k. Lets meet somewhere neutral and safe. How about we meet on a space platform so that the bad guys can't do blood magic?
GM: Sounds reasonable...

Long story short, the players knew (and I didn't) that taking magic objects into space destroys them. They had a good laugh at my expense when they explained what they did.

-Slapstick

I don't think mana warps do anything but temporarily affect foci anyway. Thousands of magical items survived the 5th Age intact and I can see no reason to play space differently. If the focus was active - ie dual-natured - at the time, maybe I could see damaging it while in space.

Based on the amount of foci being constructed in the 6th age, either many of the weaker ones did become useless during the downtime or many of them were broken and forgotten during that time while their powers were unavailable. Either is reasonably likely and both explain the small numbers of artifacts.

I think the manner of function is also relevant. Most people have no idea how to use foci created during the 5th Age. In Bug City they talk about the large number of magical items in the Sanctuary's possession due to the Field Museums collection. But it also mentions that they have no idea how to use most of them.

I would think most of the magical items of the 4th Age simply did not remain intact and thus lost their magical properties. Though, I will concede the possibility that 5,000 years in a mana lull might drain a focus to dust.

or the dragons have them all.