I have spotted a few of these cracks on older RV6's, they were always on the side or back and are not easy to spot unless you know what to look for.
Despite the owner thinking it was fine 10 hours ago I find that highly unlikely as cracks/failures like this are always progressive in nature (unless a gorilla was flying it perhaps).

The few instances of failure that I am familiar with all had a noticeable indication of a compression buckling failure as the start of the failure sequence, which then progressed to cracking along the weld and then ultimately total failure.
In this instance, there may be signs of a buckling failure but just not evident in the one photo.
One thing that is interesting to me is what occurred at the neutral axis of the tube intersections (The point where the load is transitioning from a tension to a compressive load). See attached photo.

It was likely the last area to fail and since the crack hadn't yet propagated through that area, the failure was just pure overload on the remaining un-cracked material so it just tore the material. I think that is what caused the squarish shaped areas beyond the edge of the weld. Just prior to these areas failing the loads were probably being transferred simply by the edge of one cracked piece, pressing against the edge of the other cracked piece.
(but that is just a guess... it would take a closer look with some higher powered magnification to see if there is any actual evidence of that.

As an owner of a "New to me" RV-6
 

I wonder how many failures are being experienced by new owners of legacy RV-6s. I have flown a number of airplanes that you could do jumping jacks on the rudder pedals without hurting them. And, if hadn't been aware of this SB before I bought mine, I might not have had enough appreciation for the opportunity to overload the pedal system with lead feet.
Too bad there isn't a way to have a "Tell Tale" incorporated into the system to give a heads up that an overload may have been experienced...

Question for Scott:
 

Scott, if possible can you say how much load was applied during the testing to achieve a failure?

On a side by side RV equipped with pedals that have had the gussets properly installed, damage occurring from overload is unlikely, but the "tell tail" would be the discovery of it early while properly conducting a yearly condition inspection.

As mentioned in the SB document, an un-modified assembly was found to meet the load requirements dictated by FAR 23, but that since failures had occurred, it was re-enforced anyway.

I remember that when testing a gusseted assembly, the load value at failure was substantially higher than the un-gusseted test article, but I do not remember any specific values.

No internal primer on the Torque Tube, but they are clean and show no sign of corrosion. Total time on the Aircraft is approximately 500 hours. First flight in 2001 so She's 20 years old...As to the guy who's post you like, my opinion is he's dead wrong on signs of cracking that were missed within 10 hours. The wisdom from local engineers who have looked at the part is that the weld was not "normalized" with heat and this weakened the Torque tube (crystalizing some of the internal metal) which was the point of failure, NOT the weld itself which is entirely intact. Powder coating a dark color is a very bad idea, and personally I wouldn't do it at all but the new ones from Van's come powder coated in white.

Smak

Tube not internally primed but no sign of corrosion either. Local engineers who have inspected the failure conclude that the weld was not "normalized" thereby leaving the torque tube with some crystallization and weakness. The weld is not the point of failure...The Tube is, subtle but significant difference.

I made a 90 right on Taxi out, felt something, let's say "weird" and thought it was just the tailwheel taking a bit to recenter. I made a Full Control Check just before takeoff and that's when the right pedal stayed full forward. I now believe the one side/arm snapped off with that first 90 right and the second broke loose with the control check. About 500 hours on a '96 kit completed in 2001.
A&P IA with the Absolute Best Paperwork I've ever seen had just signed off the SB on a pre purchase conditional inspection...LESS than 10 hours before failure.
Put the new pedals on!

Smak

Planned SB Revision
 

We've reviewed this as well as other past reports, and Van's Aircraft will be revising SB 99-6-1, removing the option for compliance by inspection every 10 hours. The original service bulletin was issued more than 20 years ago. Any aircraft that has not yet been brought into compliance by replacing or modifying the pedals has likely experienced a significant number of years/hours in service. Therefore, the potential is now even greater for existing unmodified parts to be weakened as a result of use and environmental factors.

Our engineering team has therefore determined that the proper course of action is to revise the SB to require either replacement of the parts with the current version, or reinforcement of the existing pedals with the gusset kit that's been available free of charge from Van's since 1999. The details are in the service bulletin document, and we plan to publish the revised version on Thursday before the close of business.

While compliance with service bulletins is technically at the discretion of the owner/operator of the aircraft, Van's Aircraft strongly recommends complying with all service bulletins, including achieving compliance by implementing the changes outlined in this bulletin for operational safety reasons.

good. issue the revision and move on.