Can anyone explain this MySQL error?

ERROR 1025 (HY000): Error on rename of '.\forumf\#sql-1718_20' to '.\forumf\posts' (errno: 150)

1. Is the error number 1025 or 150?
2. I was deleting a primary key (which I had set by mistake) which was also a foreign key at the time.
3. What is HY000?

I got this quite cryptic error message while editing keys on a MySQL table. Can someone explain the error, it's cause and it's fix?

From stackoverflow

• Here is a link from a simple googling.

  http://www.simplicidade.org/notes/archives/2008/03/mysql_errno_150.html

• Yeah, you hit a foreign key constraint.

  A show innodb status; will show the cause of the problem.

  A quick fix is to drop the constraint, do what you're doing, and then recreate the constraint.