Bit padding instead of PKCS#5 padding
With byte aligned data, bit padding allows the padding oracle attack. Every message has to end in a 0x80 byte followed by any number of zero bytes. You can iterate one byte at a time just like with many other byte paddings.

If you allowed plaintexts that are not a full number of bytes long, the attack wouldn't be possible. (Every plaintext that didn't end in a full block of zeros would decode, so no attack.) However, most real world encryption uses byte aligned data.