Going for Jungle creeps instead of the Nexus
Almost 100% of the time I lose games its because some Leona support is jungling when we are ahead, and give the enemy team time to reset, and ace our team instead of attacking the Nexus.
After that the teammates react to what is happening by rushing into the jungle to fight 1v5.
Then the enemy team kills towers, inhibs, and the Nexus.
If you are ahead, apply map pressure, or group, DON'T jungle farm.