I sometimes spot colleagues making design decisions that seem really bad to me. A common pattern is building something that already exists in much better form as open source software. I usually try to get them to realize the error themselves by asking questions like "how does this compare to open source library X" but it doesn't always work. How do you deal with things like this, keeping in mind that I'm not all knowing and could be wrong but also feel a responsibility to protect the larger organization from bad tech.
I had this problem too as a team leader.
Most of the time employees are aware of this bad decision but still want to make their own engine/library because it's more fun and they think they can get more credit by building their own library.<p>Instead of telling them what to do (nobody like it) try to give them a clear deadline for the whole project.