No one likes to hear “I told you so”. By the time you’re saying it, it’s too late.
It’s ok that you arrive at a conclusion before someone else. They’re not required to process their decisions at the same pace. But if their speed in decision making is going to cost them time, money or other risk factor, then you have a decision: deliver an I-told-you-so later, or deliver someone else now.
A proper “you’ll be sorry” now is much more useful than an “I told you so” later. The benefits of a cautiously and professionally worded warning are numerous:
you’ll rest easier knowing you warned the other person
you’ll almost always reduce the remediation cost
you’ll reduce risk and exposure to risk
One of the most selfish benefits is reason enough to do it: If you’re right, they’ll listen more closely next time.
Part of the reason you may be an IT pro is that you understand things better than others. Normally you’d expect colleagues (especially non-technical ones) to defer technical questions to you and say “you know what you’re talking about, so I’ll listen.” However, people don’t always do that.
This is a pivotal moment! That small space of time between when they disagree and you have the choice to just comply or protect yourself against any future blame.
So if you get that “they’re gonna be sorry!” voice in your head, write down the risk and what is needed to fix it. Share it with them. Be specific, don’t be melodramatic. Copy one other person at a minimum, to ensure you don’t get the dreaded “you never told me” excuse.
One more thing: take the high road. Don’t say “I told you so.” – but quietly enjoy that next time you will likely be heard and trusted more.