[MC-117664] command blocks nbt tag broken

/testfor @p {Inventory:[{id:minecraft:diamond_helmet,tag:{display:{Name:game}}}]}

[15:29:10] Data tag parsing failed: Expected '}' but got ':' at: {Inventory:[{id:minecraft:<--[HERE]

Linked issues

Since 1.12, you need to quote all strings which do contain characters which aren't alphanumeric. So, your fixed nbt query would be:

/testfor @p {Inventory:[{id:"minecraft:diamond_helmet",tag:{display:{Name:game}}}]}

you should make a command up
dater for older commands

There are so many of those in the internet.