There is more than one way to read 'willfully misunderstand' people. If we assume it to mean Elizabeth chooses the most amusing way to understand what is said, it makes the most sense.

Darcy and Elizabeth's interactions have been few enough that we can track down some of them, although, others might have been not described to us in full, especially whatever Darcy overheard during listening to Elizabeth's conversations with others.

"Did not you think, Mr. Darcy, that I expressed myself uncommonly well just now, when I was teazing Colonel Forster to give us a ball at Meryton?"

"With great energy; but it is a subject which always makes a lady energetic."

"You are severe on us."

He is not actually being very severe here, is he? But she takes it that way. Later, when Sir William wants them to dance:

"Indeed, sir, I have not the least intention of dancing. I entreat you not to suppose that I moved this way in order to beg for a partner."

No one is supposing that Elizabeth was maneuvering for a dance with Darcy, but she defends herself from the charge for the turn it gives her refusal. Darcy then offers himself as a dance partner, and Elizabeth tells Sir William:

"Mr. Darcy is all politeness," said Elizabeth, smiling.

She means here that he is merely being polite, and does not really want to dance. At Netherfield, Mrs. Bennett is going on about a former admirer of Jane's writing verses on her:

"And so ended his affection," said Elizabeth impatiently. "There has been many a one, I fancy, overcome in the same way. I wonder who first discovered the efficacy of poetry in driving away love!"

She is making an outrageous generalization here, and Darcy calls her on it, but Elizabeth defends a position that he must suspect she does not believe. Then Darcy seems to be picking on Bingley for his 'indirect boast' of changing his plans on a whim, and Elizabeth defends him, saying to Darcy

"You have shewn him off now much more than he did himself."

She does not believe that Darcy is displaying a trait he admires - it is simply funnier to say that he does. Then she inquires about Darcy's supposedly preferring rashness that leads to obstinacy over simple rashness, and when Darcy tries to answer rationally instead of treating the conversation as mere witticisms and jibes, she pretends to understand him to say:

"To yield readily -- easily -- to the persuasion of a friend is no merit with you."

During the Miss Bingley and Mrs. Hurst's piano playing, he asks her inclination for dancing a scottish reel. She eventually answers:

"[...] You wanted me, I know, to say 'Yes,' that you might have the pleasure of despising my taste; but I always delight in overthrowing those kind of schemes, and cheating a person of their premeditated contempt. I have, therefore, made up my mind to tell you, that I do not want to dance a reel at all -- and now despise me if you dare."

She did not really think Darcy was setting her up to despise her taste, nor did she think 'making up her mind' to SAY she did not want to would make anyone think that she really did not. When Elizabeth does not want to walk with Darcy, and Bingley's sisters, she says that she does not want to spoil the grouping. When Miss Bingley asks what Darcy could mean about their motives for walking the room together, she says that Darcy:

"[...] means to be severe on us, and our surest way of disappointing him will be to ask nothing about it."

When Miss Bingley defends Darcy from being teased:

"Mr. Darcy is not to be laughed at!" cried Elizabeth. "That is an uncommon advantage, and uncommon I hope it will continue, for it would be a great loss to me to have many such acquaintance. I dearly love a laugh."

She does not believe that he is not to be laughed at, and pokes fun at her own loving to laugh at acquaintances. She also pretends to readily agree with Darcy that

"[...] Mr. Darcy has no defect. He owns it himself without disguise."