Anyone can short shares in the marketplace in these corporate actions. The key to making money is to make sure you borrow the shares from someone who is an non electing shareholder (ie someone who misses the deadline).
Who better knows who the non-electing shareholders are? (and how many shares)
I've been able to do this at times. You can try yourself. Short shares of a partial tender offer. It may work. It all depends on who you borrowed shares from. It all depends on how many shares missed the deadline etc. It isn't just the result of a mistake. Sometimes large investors choose not to tender shares at higher prices because of tax consequences etc. For example, Warren Buffett would not tender shares of KO if he could sell a portion 10% higher in a tender offer. If you could borrow his shares - then you would be golden. Also, borrow prices tend to go higher for obvious reasons.
This happens in many corp actions. Rights offerings, takeovers, tender offers etc
Sometimes it involves withholding taxes (ie non electing shareholders have money withheld in some foreign mergers) and the option markets are adjusted based on non electing shares.
The NXP/QCOM merger if it had gone through would have been very profitable for many
And I'm not saying certain brokers do this - just that it is done.