title says it all :]

example

@nitrogenfive Hey! I'm assuming this is multiplayer, since I've encountered this on a server also.
As far as I know this happens when the server has experimental physics on, but the client thinks it doesn't.
This can happen when the mod is installed server-side only, and the mod config is used to force enable experimental physics, as opposed to enabling it the vanilla way, (through the More > Experiments tab at singleplayer world creation.)

As a temporary fix while this gets patched, you can try enabling the experiment on an already existing world by modifying the level.dat file in your world folder like so:

1. Grab some software that can edit level.dat files, I'm using NBTExplorer.

2. Stop your server and open up your level.dat file!

3. Create a new tag list under the Data compound tag named enabled_features!

4. Under enabled_features add two string tags: minecraft:vanilla and minecraft:minecart_improvements!
   It should look something like this:
   

5. Save the file, and the mod should work completely server-side now.

Let me know if this helps!

When trying to fix this, it may be worth taking a look at this pull request from another minecart physics mod named Audaki Cart Engine, as it was used to fix a very similar issue.

when the server has experimental physics on, but the client thinks it doesn't.

Good call, didn't think about that. Will see about integrating that fix. Thanks!