Game crashes when command in command block is too long

Your problem is you are trying to make a single command block do too much. You should not be stuffing enormous amounts of code into a single command block. They have limits for a reason. If your "Mod in a block" is crashing with one command block then maybe you should use two? Just like when a score in the game reaches its maximum value and then sends said score into the negatives, because there is a limit. You have a 32,767 character limit that should be more than adequate for any use. If not USE TWO.

If you believe it or not, I know how to make a command block mod. But it's a bug that the game crashes and that should be fixed.
By the way: At the moment I'm just doing a update for 1.8.6 and I've already a second command. I will find a possibility for sure.

I am not saying you don't know how to, but what you are doing is stuffing TOO MANY commands into a single command block. Trying to use a single command block like a .zip file. When you activate that 1 block in unpacks the rest. so actually it is NOT a SINGLE COMMAND mod, but a neatly packaged mod, which is ok. What I was trying to say is that it is NOT a bug they gave a command block a set character limit, you went over it and crashed your game. Stay UNDER the LIMIT and you will be fine, if you need to go over 32,000 characters use two.

If you ask me the game is working as intended, as the command blocks do have a set limit. Though if there is any bug it is that the game allows you to add characters over that limit and then crashes, an easy fix would be to make the command block stop input at 32,767 charaters, as it is the limit.

I'm not saying that it is a bug that I can't enter more characters to the command block (well, it would be a nice thing, tho). The bug is that I can enter more characters to the command block than minecraft / java can analyze. And if the game can crash, this isn't intended if you ask me. That's the same thing as if you build a calculator that crashes when you're trying to divide by 0. The fun thing about that is a command block actually has a limit were you can't add characters anymore. To the command from the text file above you can only add 3 more characters.
Yes, I would call it "mod that can be entered with only one command" or something like that as well, but I've seen others shorten it to "one command block mod".

It always irked me when people say "SINGLE COMMAND MOD!" when it is really a packaged mod, containing many command blocks/commands. But anyways, I did not know that the command block will stop accepting characters at a point. I hadn't tested that limit, I only tested to the point of crashing.

A really "only one command mod" is for me turning arrows into ghast fireballs.
You only need a /execute @e[type=Arrow] ~ ~ ~ /summon Fireball ~ ~ ~ {direction:[], ExplosionPower:1}
And than let this run in a comparator clock. This idea is from SethBling.

Usually when I post a mod in a forum I write "Enter it with only one command". I actually don't like "only one command" as well, but it doesn't anger me. Well, for the bug report it was enough for me to use the shorten version for some reasons I really can't explain.

But let's stop the offtopic 😉

It's Java having issues here, not Minecraft.
Java allows Strings up to a size of 32768 bytes and no more.
If this really is a problem to you, report it to Oracle.

Try making a Java class and set the value of a string to a length of >32768 to demonstrate, it won't compile even.