Well we've enough accounts of it certainly to believe that it occurred, Quinctilius Varus is said to have

The general had more courage to die than to fight, for, following the example of his father and grandfather, he ran himself through with his sword.

According to Velleius Paterculus who is probably one of the best sources on the battle. Cato for example seemed to have messed up doing it according to Plutarch.

As best i understand it on the details as a method of suicide its probably not precisely what you're thinking of. Obvious warning for those of you with a slightly weaker stomach. When referred to in a greco-Roman context in particular the action is different to that as part of Japanese Seppuku. You line the point of the sword up just bellow your rib cage, round about the solar plexus area, lining it up so its pointing upwards and inwards. Then you can use both your body weight and arm muscles to drive it upwards under the ribs into the chest cavity, ideally you'd at least nick the heart. That kind of injury if done correctly would be bloody but quick, once the heart is punctured the blood loss and shock resultant is extremely rapid. Pretty much instantly someone's eyes lose focus and they're unconscious pretty quick after that and dead shortly there after.

On methods Plutarch says on Cato's death, again if you do not want to hear the explicit details of a man's gruesome and painful suicide don't read this

But when Butas had gone out, Cato drew his sword from its sheath and stabbed himself below the breast. His thrust, however, was somewhat feeble, owing to the inflammation in his hand, and so he did not at once dispatch himself, but in his death struggle fell from the couch and made a loud noise by overturning a geometrical abacus that stood near. His servants heard the noise and cried out, and his son at once ran in, together with his friends. 6 They saw that he was smeared with blood, and that most of his bowels were protruding, but that he still had his eyes open and was alive; and they were terribly shocked. But the physician went to him and tried to replace his bowels, which remained uninjured, and to sew up the wound. Accordingly, when Cato recovered and became aware of this, he pushed the physician away, tore his bowels with his hands, rent the wound still more, and so died."

Plutarch says that his blow was feeble on account of the hand wound but i think based on the description this is also at the wrong angle to fall on someone's sword properly. Possibly his hands was shaking too much and he didn't get it in the way he wanted to but also perhaps the gory details of the right way to do it wasn't sufficiently common table talk that Cato could have been mistaken about how to do it properly. Certain when people heard of the way Cato died back in Rome it was considered shocking that a man such as himself had died in such a way when Caesar's triumph displayed paintings of the event a while later.