I find it's the chip readers that are very flaky. Certain stores I go to simply have unreliable chip readers and I often have to fall back to the stripe. This has even happened at a couple places right after they replaced their readers. It seems to be related to alignment of the card in the slot, which has led me to a procedure that on the second try, I'll put the card in while sliding it pressed against the left edge of the slot, and on the third try I'll slide it in on the right edge of the slot. This small bit of offset often makes the difference.

I suppose after the stripe goes away, they'll have to improve these machines in a hurry. I just don't understand how they're so shitty and yet the rest of the world seems to be using them without issue.