The first four bytes of transaction data identify which function is being called. Here is how to work out what an unrecognised one means.
Every contract call begins with four bytes: the function selector. It is the first four bytes of the keccak-256 hash of the function signature — `approve(address,uint256)` hashes to a value beginning `0x095ea7b3`, so that is what an approval starts with.
The hash only runs one way. You cannot turn a selector back into a name; you can only compare it against names other people have published. That is what every decoder, including this one, is doing.
So an unrecognised selector is genuinely ambiguous. It may be an ordinary custom function in a perfectly honest contract, which is common — or it may be something you have no way to inspect. The honest answer is that it is unknown, and any tool that presents a confident guess is misleading you.
Four bytes is also short enough that two different functions can share a selector, and short enough that a determined attacker can search for a name that collides with a familiar one. This is rare, but it is why a decoder should show every candidate rather than silently choosing.
If you did not expect an unusual function call, an unknown selector is a reason to slow down rather than a reason to panic.
Check a transaction of your ownPaste anything. It is decoded in your browser.