Absorbed Android 17 and gained all of his power, giving him infinite stamina

"Ah, 17, how brave of you," said Cell, licking his lips. "I guess you'll do just as well for now." Faster than anyone could see, the creature's clawed fist shot out, catching 17 across the jaw and knocking him completely senseless. Within moments, Cell's tail expanded at the end, enveloping the cyborg's unconscious body.

Vegeta smirked, glad that the deed was finally done. Even as the helpless Number 18 wrestled, struggling to break free of his grip, she wept. Her brother was gone, taken forever. Goosebumps gripped her skin as the true fear of being alone hit her; she was the last android left.

"NOO!" shouted Trunks, taking off toward the creature as fast as he could. But it was too late. Cell's body began to glow, and energy erupted around him, flinging Trunks away. The transformation had begun.

- Bringer of Death Chapter 44

Several brave warriors, protectors of the earth, observed all these events in stunned silence. The creature they had thought to be Goku mere hours ago was transforming into an even worse monster, the most powerful they could have ever imagined. Each Z fighter covered his eyes, struggling with their energy to resist the pull of Cell's aura, still swirling around him. His power…somehow it continued to rise above all expectations.

"How is his power still increasing?" the Namekian named Piccolo shouted. "The android's power didn't just increase Cell's—it's like Cell's getting ALL of it!"

...

Cell sneered. "What you're seeing before you now is my Semiperfect form. Not only are my abilities greatly improved over that former...imperfect...form, but I now have all of Number 17's power at my disposal. He has joined me in pursuit of Doctor Gero's perfect design for the ultimate warrior."

- Bringer of Death Chapter 45

Q: Vegeta should have had a hunch about cell's power increasing above his, when cell's imperfect form already outclassed his super saiyan form.
A: Indeed he should have...but he was tricked. Remember, the cyborgs don't have detectable energy signatures, so it would be difficult for any of the Z Fighters to know exactly how strong they were. Vegeta knew that he was much, much stronger than he had previously been...and so he didn't think it through all the way. He never imagined that Cell would be able to add all of 17's power to his own.

- Bringer of Death Chapter 46

Standing and brushing dirt from his shirt with two pats, 17 smirked up at Vegeta. "You honestly think you're winning, don't you? Maybe this time you're the one who should be paying attention. Haven't you noticed yet? My power is infinite—I never tire out. But you, you can't keep this up forever. Face it, Vegeta, you're through."

...

Vegeta stared silently at 17's blast as it sped past him, close enough for him to spit at it. Apparently, the machine really did have perpetual energy—a stark contrast of brilliant ingenuity compared to the weak absorption types which seemed to be constantly losing even the ki they had stolen from others. Yet even as he watched the yellow stream of photons crash into the sea behind him, the explosion sending walls of water hundreds of feet high into the air, the prince smiled—this was a worthy opponent after all.

- Bringer of Death Chapter 38