As far as I know, the only instance of a poison pill being triggered was Versata Enterprises, Inc. in December 2008. TL;DR: A competitor (Selectica, Inc.)--wis

As far as I know, the only instance of a poison pill being triggered was Versata Enterprises, Inc. in December 2008. TL;DR: A competitor (Selectica, Inc.)--wishing to purchase Versata--bought over 5% of the shares. The IRS considers this an ownership change, and it imposes a penalty to discourage trading of Net Operating Loss (NOL) carryovers. A poison pill was in place to prevent this tax issue. The board executed the poison pill to dilute Selectica ownership from 6.4% to 3.9%, and that action held up through appeal to the Delaware Supreme Court. https://www.lw.com/upload/pubcontent/_pdf/pub2563_1.pdf https://www.lw.com/upload/pubcontent/_pdf/pub2563_1.pdf