For example, although Superman has demonstrated that he had enough Superhuman Strength to push a planet or asteroid out of its' natural orbit in space, this was still something that was very difficult for him.
On one occasion, a planet made of Green Kryptonite was on a collision course for Earth, and Superman decided that it was a job for him to stop it, and he said "And an easy one at that!" Despite that he was unaware at that point that the planet was made of Kryptonite, he still must have been joking, since other accounts have shown that moving a planet was a difficult task, even for him.
This showed that even characters with such an incredible power level as Superman had his limits. Even Hal Jordan, one of the greatest Green Lanterns in the Green Lantern Corps, had limits, despite the fact that his Green Lantern Ring was fueled by his Indomitable Will. Clearly, even his will wasn't completely "indomitable."