I would pick one recent episode, the CVE-2018-17144, the latest found Bitcoin vulnerability that could have crashed the network. It was reported by awemany, one developer of Bitcoin Unlimited (Bitcoin Cash).
As far as I know, awemany while testing BCH found one error and decided to test it with BTC which he found that it had the same error. Well, he took the initiative to test BTC, he lost his time and he reported the issue.
What happen after that, the disrespect, the opinions that came up concerning the quality and competence of Bitcoin Cash developers, it contains much of this "Bitcoin maximalism" spirit underneath.
Cooperation should be appreciated, any altcoin developer who contributes and do a good job should treated professionally and with respect. People should be motivated to contribute more.