Dave Weigel notes the underreported fact that Frank Lautenberg hasn't been healthy enough to vote for some time now, and that his missing vote could scuttle the gun bill. And that this matters, in part, because the Senate did not adopt the flipped responsibility on cloture -- one of the relatively minor reforms that was proposed but rejected in January was changing the cloture rules from 60 votes needed for cloture to 41 votes needed to prevent cloture.
Now...on the other hand, I'm really not convinced that it's going to matter in this case. A bill (or key amendment) that can't get 6 Republicans (assuming all 54 other Democrats vote and vote for the bill, which is hardly certain) is probably doomed in the House, anyway. If I had to speculate, I'd guess that the gun bill needs at least 10 of the 45 Republican Senators to stand a decent chance in the House. Although I suppose there's a possibility, for example, that a Senator could vote against Manchin-Toomey but still vote for cloture on the final bill...the final bill probably needs strong support to have a chance in the House, but perhaps there may be important votes that really need "only" 60.
Still, the logic of 41 to sustain cloture vs. 60 to defeat it is awfully strong...although there's no particular reason that the rules on this should be, well, logical. After all, the 3/5 standard is arbitrary to begin with -- there's a logic of sort to supermajority, but not really for this particular supermajority. Also: remember that switching to 41-to-sustain would not lead to any other particular burden for the minority; in particular, it wasn't going to lead to surprise cloture votes to try to catch the minority with Senators too far from the Senate floor to vote.
At any rate: nice catch!