Like, if I use a resource pack for 1.14... on 1.12.2 will it do anything wrong?
It depends on which version you're going from and to. This can vary in intensity from a few GUI elements not showing up (or not showing up in the correct places), all the way up to the pack outright crashing your game.
In that particular example, there is a LOT that could potentially break since 1.14 introduced a lot of changes that aren't backwards compatible. Depending on the pack it's very likely that a lot of textures just won't work. They might remain default despite normally being in the pack, and they might instead be purple and black checkerboards. Crashes are unlikely but still possible.
It's always best to use the correct version of a resource pack for the version of Minecraft you're playing. You can always try it and see what happens. Worse case scenario is that you just need to delete the pack from your /resourcepacks/ folder to stop the game from crashing, though it likely won't be quite that severe.