Version mod: Immersive Engineering 0.12 - 89
Version forge: 1.12.2 - 14.23.5.2838
Description: Player placed block and have this crash on client.

log.txt

What block did the player place? If they somehow got the excavator block itself (only possible using cheats//give), then it is (in my opinion) their own fault for cheating in blocks they don't understand. Also, what launcher are you using?

What block did the player place? If they somehow got the excavator block itself (only possible using cheats//give), then it is (in my opinion) their own fault for cheating in blocks they don't understand. Also, what launcher are you using?

He building excavator and activated his then he get crash client.

I'm unsure how to reproduce or safeguard this.
If you can't provide a reliable test case in an environment with just IE, I'm afraid there's nothing to do.