Encrypting 256 bit AES keys with AES ECB?
If you are encrypting a 128-bit key, then it's "OK" since the key size is the same as the block size. However, if you encrypt a 256-bit key, then it's possible to tamper with the result. For example, you can drop one of the blocks and it "looks like" a 128-bit key, or you can make a 256-bit key be the same block twice (so it's of the form $k\|k$ where each $k$ is 128 bits, and so on. 

Having said the above, even in the case of 128-bit key, you have the problem that an attacker can tamper with the ciphertext and you won't be able to detect it. It is true that the result will be random garbage, but in most cases, we like to be able to detect the tampering.