Unpluging mounted shared dir - will it crash the filesystem?
 

For example we have a PC with Linux and, let's say, ext4. It is connected to another PC with Linux and Samba-shared dir.

First PC mounted shared dir of the second one. So it's in the filesystem, for example in "/mnt/000/". What will happen if I unplug the net cable from first PC? Will ext4 on the first PC crash so I'll have to perform fsck? I know that hot unpluging of mounted HDD probably will make filesystem read-only available and generally damaged. What about the case I described above?

No, the client computer will probably churn for awhile and spit out some errors if you try to access the share but nothing will happen to the server.