Since 1955!

I agree.

Unfortunately, after reading all books and watching all TV episodes, my memories are sparse. So, I'm not 100% sure if this happens on both book and TV or books are "fair". But I believe these are examples:

>!Poirot had telegraphed her secretary trough her discovered that 2 people got married, only reveals it at the end.!<

>!Poirot knew the exact train timetable and was able to make deductions based on that.!<

>!In at least a couple of stories, events were related to an accident happened years before that was not fully presented. (Murder on the Orient Express)!<

>!In Curtains... well, you know what Poirot did.!<

Then, there are obstacles like very specific knowledge that a reader is unlikely to have. For instance, knowing how a poison works or the currency used in a foreign country.

All in all, I believe Christie's stories are usually not solvable while reading the book. But they're always enjoyable. I like the richness in details and I find the characters more relatable. I have not enjoyed much Conan Doyle stories, to the point I feel the most famous detective should be Poirot instead of Sherlock Holmes.

Apologies for my vague examples, I'm happy to edit and add references if someone can remember which book they happen on.