I was trying to customize a potion using the CustomPotionEffects DataTag and the /summon Command - here's the command I used:

summon Item ~ ~2 ~ {Health:5,Age:0,Item:{id:373,Count:1,Damage:8257,tag:{CustomPotionEffects:[{id:1,Amplifier:10,Duration:400}]}}}

So summoning the item worked just fine, the potion landed right where it was supposed to and I was able to pick it up. But when I opened my inventory and tried to hover over the potion with the mouse cursor, the game instantly crashed. This is what I got as a report: http://pastebin.com/mXJGcLG0