Here are some examples:
1) As
@plawolf had pointed out. Had the PRC been in the P5 since the formation of the UN. Then it could have vetoed the UN resolution to intervene in the Korean War. Instead, that lack of veto power forced the PRC to intervene militarily into the Korean War to protect its own national security. That was one instance where veto power might have stopped a major war that had killed millions.
2) Russia and China had allowed a Western-led UN resolution to impose a no-fly-zone over Libya in 2011 for "humanitarian purposes". It got unanimous UN votes to pass. At the end, Gaddafi was liquidated and Africa was worse off without him in charge of Libya.
3) The West had tabled hostile UN resolutions targeting Syria. Luckily, Russia and China had learnt the lessons of Libya and used their veto powers to block them. Had that not been done, Syria would surely have gone the way of Libya too.
4) During the Ukraine war of 2022. A number of resolutions to condemn Russia got sufficient UN votes to pass and Russia vetoed them. Clearly a good portion of UN member states didn't grasp the true nature of the Russia-Ukraine War.
5) China has vetoed India's ascension into the Nuclear Suppliers Group because India is not a signatory of the nuclear NPT. China was enforcing the true purpose of the NPT. But too many UN member states didn't mind ignoring the NPT for India.