When characters jump between campaign worlds being run by different DMs - as has become commonplace in FLAILSNAILS circles - that travel raises a lot of questions, even if you handwave the 'how' of it. Mike at Terruizeng raises some of those questions here, and they're worth thinking about.
Focusing on the tech-transfer aspect, linking up campaign worlds means you have to deal with higher tech levels (or higher magic levels) leaking into lower-tech (or lower-magic, you see the pattern) worlds. As a DM whose setting includes firearms, this is something which concerns me. Now, the FLAILSNAILS conventions include a very simple 'out' for those DMs who don't want ray-guns and stuff showing up in their game on the hips of visiting PCs. You just say 'no': you don't have it on you (you'll get it back when you 'port out, one presumes), or you have it but it functions differently or not at all.
And that 'no' is all well and good...but can't we find a way to do this that adds to the game in a meaningful way? No, not a life-affirming meaningful, but something that adds cool or generates plot hooks or something? I think we can.
Here's a draft of a little table to determine what happens to "banned" items when their owner jumps from one world (the 'old' world) to another (the 'new' world). The table presumes that you're looking for some kind of transformation of the item rather than "it disappears" or "it works fine" (you don't need a table for those, I hope). As I'm drafting this, I'm primarily thinking about guns, but it could be applied to anything with some wiggle room.
CROSS-DIMENSIONAL LOOT PARADOX TRANSFORMATION TABLE
Item transforms into...
1 - sand
2 - a (poisonous?) serpent
3 - a replica of itself in stone
4 - a hunk of driftwood that vaguely resembles its former shape
5 - a handful of leeches
6 - small swarm of bees or wasps
7 - an oddly-colored crystal
8 - a mysterious key
9 - a map to hidden treasure(?) (1-3 new world, 4-6 old world)
10 - a bottle of unusual wine or liquor (origin: 1-3 new world, 4-6 old world)
11 - a rather sizeable pearl
12 - fragrant dung from an unknown creature
13 - a mysterious letter, from/to parties unknown (DM's option: letter is 'to' a PC, or even better, 'from' a future version of a PC)
14 - mummified hand (1-2 simian, 3-5 humanoid, 6 seemingly demonic)
15 - functionally equivalent new-world item, but better (ie, revolver turns into masterwork hand crossbow)
16 - large egg of strange hue (contains 1-3 creature, 4-5 liquid silver or gold, 6 non-sequitur (ie, a pocketwatch, a foil-wrapped chocolate bar, collection of tiny poetry scrolls)
17 - 'passport stone' - a small ceramic disk with seven sections; one section is a different color. As it is examined, a second section changes color; these wedges represent different campaign worlds (the old one, the new one). What happens when all seven sections are 'activated'?
18 - small box of chocolates or petits-fours which have the effect of micro-potion doses; no 'which is which' insert is included, sadly
19 - micro-figurine of less-than-wondrous power which can be activated once per day and accomplishes a mundane or unimpressive task (ex: cobalt echidna which will lace up your boots for you; amber vole which cleans your ears or teeth)
20 - velveteen bag of strange seeds