I believe that Jem may have lost his temper more than twice in the Harper Lee novel, To Kill a Mockingbird, but three examples are listed below.
MRS. DUBOSE. Atticus forced Jem to read to the irritable Mrs. Dubose after he damaged her shrubs, and Jem eventually found it a tolerable experience. But one night Atticus is summoned to her house, and when he returns, he informs Jem that the old lady has died. She has remembered Jem, however, with a final gift: a box with a white camellia--a Snow-on-the-Mountain.
Jem's eyes nearly popped out of his head. "Old hell-devil, old hell-devil!" he screamed, flinging it down. "Why can't she leave me alone?"
CEMENTING THE KNOTHOLE. After receiving several groups of gifts in the knothole of the Radley tree, Jem and Scout decide to thank Boo with a note. But when he goes to deliver it the next morning, he finds that the hole has been cemented. When he asks Mr. Radley about it, Jem is told that the "Tree's dyin'. You plug 'em up with cement when they're sick." But when asked later, Atticus declares that the tree looks perfectly healthy.
Atticus left us on the porch. Jem leaned on a pillar, rubbing his shoulders against it...
He stood there until nightfall, and I waited for him. When we went into the house, I saw he had been crying...
THE VERDICT. To Jem, it was plain that Tom Robinson was innocent and that Atticus had done his job well. But the jury thought differently.
It was Jem's turn to cry. His face was streaked with angry tears...
"It ain't right, Atticus," said Jem.
"No, son, it's not right."