How to reproduce:
1. Execute a command with string tags like that:

/summon Ozelot ~ ~1 ~ {Riding:{id:"Creeper"}}

2. Put this command into /setblock command:

/setblock ~2 ~ ~ 137 0 destroy {Command:"<put your command here>"}

You will get a command like that:

/setblock ~2 ~ ~ 137 0 destroy {Command:"summon Ozelot ~ ~1 ~ {Riding:{id:"Creeper"}}"}

3. Try to execute the /setblock command. It will conflict because one string tag overlaps another one.

It'll be better if command like /setcommand exists; this command should set a command into a command block on spicified coords. It will be so useful for mapmakers.