Be Productively Pessimistic
The earlier you can catch problems, the better. So why not try to catch them before they even happen.
When talking through plans for a project, take some time to be pessimistic. Think about what can go wrong. Are there unknowns in the implementation that could prove to be big time sinks or high risk? Are there dependencies on other teams that could fall through? Is it likely that someone will make major changes in shape or scope while the project is in progress?
Imagine something went wrong with the project and try to predict what caused it (this is sometimes called a "premortem"). It's better to bring these up early than to pretend that everything will probably be ok. The "productive" part of being productively pessimistic is then taking some time to work out ways to handle these potential problems.
Don't just be needlessly negative about things, but don't let yourself be blindsided by problems you could get ahead of with a little pessimism.