When putting down a blueprint with rail from beautifull bridge, this mod randomly try to put down ghost of water tile (the one from sandbox tool mod in my game). i'm playing KSE but I don't think it's matter in this report.

I'll take a look, probably some kind of bug in how I handle water fill.

Can't really replicate it, for me the bridges work just find with a generic waterfill mod and I can't find any sandbox tool mod so I can't test that, I need to know what beatiful bridge mod exactly your're using and a link to the sandbox tool mod your using. Also what are you trying to place the blueprint over? If it's on land or water or anything where the bridges can actually be build it should just instantly convert to the normal ghosts as soon as the water Ghosts get updated without even trying to place ghost tiles, if it's placed on something it can't be build it finds the whatever landfill/waterfill it can be placed on on puts that under the entity.

The sandbox mod:
https://mods.factorio.com/mod/blueprint-sandboxes
Beautifull bridge:
https://mods.factorio.com/mod/beautiful_bridge_railway
It's happen on all terrain it seems, even on water, but not on the whole length of the blueprint nor on the same position each time. I saw actually a few tiles ghost create while building the rails but some are but before the building too. I will try to recreate and capture the problem this week end since I'm not currently building rail. Will keep you update

Well, the water tiles are placed where some stuff need to be removed (like tree & rock). A quick exemple is available here:
https://youtu.be/xKkPsxbUbPk

My three cents guess: when checking if a ghost can be replace, if it's not possible and not linked to water, it try to put down water tiles. No idea why tho.

I was missing a check to see if the entity can already be place on the tiles it's on, should be fixed in 0.12.2 which should be out as soon as I upload it.

Thanks. awesome works by the way. Best QOL mod for factorio for me :)