(A 'red herring' is a misleading clue or piece of information which is or is not intended to be distracting)

You don't need me here yet here I am
Like a piece of junk mail in your trash can
A ruse to confuse your best-laid plan
I am a red herring

A spanner in the works
A fly in the ointment
Focus on me
You'll find disappointment
Neither on the fish counter
Nor swimming in the sea
I'm a wild card
Thrown in for free

Popping up when least expected
Ensuring attention's misdirected
Sometimes considered, frequently rejected
I am a red herring

Muscling in on the action
Bamboozling you
The ultimate distraction
A random x-factor
In an equation
Gatecrashing parties
On every occasion

When will I next appear?
There's no way of knowing
I'm not even supposed be in this poem
Just a sly stowaway
Who's come out to play
I am a red herring