Players getting kicked from a modded server for a mod mismatch

What the server is telling you

Clients disconnect at "Joining world", or with a list of mods in the kick message. Forge compares the client's registry and mod list against the server's, and refuses anything that does not line up.

The rule that resolves most of these

Mods fall into three groups, and people routinely treat them as one:

• Client-only — shaders, minimaps, zoom, HUD and animation mods. These belong on clients and must not be on the server. Putting one there causes a different crash entirely; see client-only mod on the server.
• Server-only — performance and management mods. Fine on the server, must not be required of clients.
• Both — anything adding blocks, items, entities or recipes. These must match exactly, including version.

A client-only mod is not something to "add to the server so it matches". Mark it client-side in your pack config instead.

Read the rejection list properly

The kick message names what disagreed. Two patterns worth knowing:

• Same mod, different version — the most common cause by a wide margin. Someone updated one side. Versions must match exactly, not merely be compatible.
• Present on one side only — decide which group above it belongs to, then add or remove it accordingly.

After changing the pack

Everyone must have the same pack version. The usual failure is one player who did not re-download. Bump the pack version so the launcher forces an update rather than relying on people to do it.

See the full mod-rejection write-up.

Doing this automatically

Forge Server Doctor performs the whole procedure above — reads the report, finds the real cause under the wrappers, skips the vanilla frames, and names the mod jar. 24 failure modes, each with its fix. The free edition covers the six most common startup failures.