Last year I planted 2 kinds of jalapenos and some sweet banana peppers. They were all next to each other in pots. I was surprised and confused that the "sweet" peppers were spicier than the jalapenos.
Recently I planted some seeds for this summer. I bought some seedless watermelon seeds and the package says they need a "pollinizer". I had no idea what that meant so I looked it up. Since seedless watermelons are sterile they need regular watermelons growing in close proximity so they can be cross-pollinated.
Then I suddenly realized... so that's what happened to my peppers. The sweet peppers had cross-pollinated with the jalapenos. Science!