Panna cotta is perfectly paired with fruit, but not all fruit is good for panna cotta -- the problem relates specifically to the gelatin. Enter bromelain.
Bromelain is an enzyme found in certain fruits including pineapples and kiwi. Gelatin is a protein -- specifically, collagen. When bromelain and gelatin meet, the enzyme does what it does best; it gets busy breaking down the protein into its basic amino acids. This is great for tenderizing a steak, but it's not so great if you want your gelatin to set into a smooth, solid panna cotta.
If you must have pineapple with your panna cotta, there's a little trick to avoid the problem: Heat bromelain-rich fruits (or used canned varieties, which are heated during the canning process) before using them with gelatin to inactivate the enzyme.