In Part 1, Chapter 3, Sebastian writes to Charles, summoning him to Brideshead and claiming that some sudden disaster has struck. Charles worries that Sebastian's life is in danger and makes haste to the Flyte family's estate, only to learn from Julia that Sebastian's "emergency" is a slight ankle fracture. The following is a prime example of situational irony:
“Didn’t he say? I expect he thought you wouldn’t come if you knew. He’s cracked a bone in his ankle so small that it hasn’t a name. But they X-rayed it yesterday, and told him to keep it up for a month. It’s a great bore to him, putting out all his plans; he’s been making the most enormous fuss [...]."
[...]
“How did he do it?”
“Believe it or not, playing croquet. He lost his temper and tripped over a hoop. Not a very honorable scar.”
The dangerous scenario Sebastian called Charles up to Brideshead for was not life-threatening: he suffered a minor injury. It is telling that, in Sebastian's mind, the threat of a few weeks' boredom is tragic and insufferable. This instance of situational irony also reveals Sebastian's adolescent self-centeredness. He cares little for Charles's panic and stress, focused only on his own woes. If Charles did not love Sebastian, he might view such melodrama as childish rather than charming.
In the latter half of Part 1, Chapter 4, Sebastian and Charles abscond to Venice for the summer to visit Lord Marchmain and his mistress. During this time, the two boys live life in idle repose, spending Lord Marchmain's money and doing as they please. While recounting this time spent together, Charles recalls a comment Sebastian made about the Colleoni statue, invoking dramatic irony for the reader:
The fortnight at Venice passed quickly and sweetly—perhaps too sweetly; I was drowning in honey, stingless.
[...]
I remember Sebastian looking up at the Colleoni statue and saying, “It’s rather sad to think that whatever happens you and I can never possibly get involved in a war.”
Sequestered in Venice with all manner of creature comforts, Sebastian ignorantly asserts that it is "sad" to think that neither he nor Charles will ever be involved in war. Readers know this is not true, given that the novel starts with an older Charles commanding troops. Sebastian's comment reflects both his hedonistic innocence and his romanticized perception of wartime.
Sebastian's offhanded statement about the impossibility of future wars is naive. It is also deeply tragic—the sheer scale of human loss that occurred during World War I was unprecedented, shocking all of Europe. To imagine another such war occurring within 20 years of the first would horrify anyone, let alone a young adult who has barely spread his wings. Sebastian's comment is naive, yes; but it is also wishful thinking. He wants to return to a pre-WWI, idealized vision of wartime glory, and this leads him to overlook the horrors that come along with this supposed glory.
During his first year at Oxford, Charles finds himself ensconced in a friend group that includes both Sebastian Flyte and Anthony Blanche. The latter boy is infamous amongst Oxford undergraduates, so much so that Charles’s cousin Jasper warns him against associating with Anthony. Given the intense scrutiny Anthony’s peers subject him to, what occurs after he leaves Oxford is a prime example of situational irony. Note the following passage from Part 1, Chapter 5, which brings this irony to light:
Anthony Blanche’s set broke up and became a bare dozen lethargic, adolescent Englishmen. Sometimes in later life they would say: “Do you remember that extraordinary fellow we used all to know at Oxford—Anthony Blanche? I wonder what became of him.” They lumbered back into the herd from which they had been so capriciously chosen and grew less and less individually recognizable.
Those who had once scrutinized Anthony Blanche, making him an object of humor behind his back, now fall apart in his absence, losing the social cohesion his presence provided. Anthony goes from being a pariah to a curiosity. It is precisely this dull drudgery and obscurity of Anthony Blanche's former "set" that Sebastian, and Anthony himself, are desperate to avoid in life.