During the 2017 World Series, Yu Darvish pitched poorly against the Astros in Games 3 and 7 — this after dominating both the Diamondbacks and Cubs in previous postseason series that year.
After much discussion, the consensus was that Darvish was tipping his pitches and the Astros had figured that out.
With the current revelations about Astros sign-stealing methods — methods that were allegedly in use during the 2017 season — Darvish posted a YouTube video discussion this and other related topics (note the Cubs cap on the table behind him):
In this Reddit thread, a user posted an English translation of Darvish’s video which is worth reading.