I really don't see this as a "lapse."
Fed does this a lot. A LOT. He very rarely loses the first set, but once he has a lead, he often lets up a bit. He had scads of matches last season when he got out to one or two set leads, then gave up at least a set, before shrugging, getting on with it, and winning routinely when he needed to.
The only time in recent memory where it came back to haunt him was in the Master's final, where he followed the exact same pattern -- get up a couple sets, let up, allow the opponent to catch up, then finish him off. Except this time, after all but "finishing off" Nalbandian, he DID fall back a second time to lose the match.
I think it was a bit of a wakeup, but all it probably means to him is that he really DOES have to concentrate in that final set against a good player in order to dispatch of him with very little final-set drama. Letting a player creep back into the match, only to handle him routinely in the last set -- like he did with Haas yesterday -- is just business as usual. Fed may take players and matches less seriously after he gets up quickly and easily, and he may let them back into the matches, but so long as his head is where it needs to be, these players pose no real threat once Fed decides to finish the match.
That's what I saw in the Haas match. A fun matchup with lots of good tennis, but no time when Fed was actually in any danger of losing. He's fine.