The 2 most dramatic:
1. Scouting unlocked roaming special forces, which destroyed a home command station and started a cycle of massive raid -> lost home station/core station -> AIP spike -> new, BIGGER raid. By the time we finally lost, we had 4000 ship raids coming in (diff 9 experimentalist ai + 260 AIP less than 2 hours in).
2. wiping out a tier 4 adjacent to my homeworld, only to find upon colonizing that the next world out was a tier 3 (and which could actually spawn tier 3 ships, since it wasnt directly adjacent to the homeworld). THAT was a mistake... tier 1 ships vs 600+ tier 3 ships (and probably 250 bomber 3s) with no substantial defenses = quick way to lose.