Conversation so far:
When did Roger Clemens join the Red Sox?
1986.
How did he perform in his first season on the Red Sox?
The 1986 ALCS clincher was Clemens' first postseason career victory.
How much was he paid by the Red Sox?
CANNOTANSWER.
How long was Clemens on the Red Sox?
1995.
What team did he join after the Red Sox?
CANNOTANSWER.
Why did he leave the Red Sox?
Clemens greatest postseason failure came in the second inning of the final game of the 1990 ALCS against the Oakland Athletics.

Question: What was his failure against Oakland in 1990?
Self-contained rewrite: What was Roger Clemens' failure against Oakland in 1990?