Submitted by zzzitron
The safeTransferFrom does not comply with the ERC1155 standard when the token is sent to the old owner.
According to the EIP-1155 standard for the safeTransferFrom:
Lets say alice does not hold any token of tokenId, and bob holds one token of tokenId. Then alice tries to send one token of tokenId to bob with safeTranferFrom(alice, bob, tokenId, 1, "").  In this case, even though alices balance (= 0) is lower than the amount (= 1) sent, the safeTransferFrom will not revert. Thus, violating the EIP-1155 standard.
It can cause problems for other contracts using this token, since they assume the token was transferred if the safeTransferFrom does not revert. However, in the example above, no token was actually transferred.
Revert even if the to address already owns the token.
jefflau (ENS) confirmed and commented:
LSDan (judge) commented:
