How PoS Blockchains prevent miner from sending different block

[flow_]

Hello everyone,
I'm quite new to the world of crypto and have been wondering, how different blockchains prevent miners/forgers from breaking it.
As far as I know, (in PoS, for instance) a forger get's selected to validate a block of transactions.
But theoretically speaking, he could create different versions of the new block he is validating.

Example:
The miner/forger received 100 transactions and is including all of them in his block A.
Also, he creates a block B that only holds 95 of the transcations.
Now, he broadcasts block A to 50% of the network and block B to the rest.
Both blocks are valid and are signed by him -> he created two different blockchains.


What am I missing / how are do blockchain projects adressing this issue?
Thanks.

[joniboini]

What you describe is essentially the nothing at stake problem. Several solutions have been proposed to solve this problem. You can give a validator a penalty if he votes on every fork/chain that happens on the network. If he votes/validate on chain A and B (different blocks), he'll lose his entire stake. In addition to this, you can require that every validator identity must be known and trustable, therefore when something like this happens you can know who's at fault. In its simplest sense, the solution lies in preventing validator to validate on more than 1 chains and to discourage validator to do something like that by making his identity (real identity) known to the network.