-19 Max Health becomes -18

If you have a custom item which gives you -19 Max Health your health will become 1 heart (2 health) when you die instead of 1/2 a heart (1 health).
Use the command

/give @p minecraft:diamond_chestplate 1 0 {AttributeModifiers:[{AttributeName:"generic.maxHealth",Name:"Test",Amount:-19.5,Operation:0,UUIDLeast:1l,UUIDMost:1l,Slot:"chest"}]}

I have also found this works with any item giving/making you lose an odd amount of health (a.k.a. health where you get halves of hearts).