I purchased 4oz Avent baby bottles (Amber colored) and they leak when feeding baby. I looked online and this was a known issue. I called Avent and they said they'd replace bottles with 9oz newer version that has an additional part, a ring that goes over the mouth of the bottle, that is supposed to prevent the leaks. 2 weeks later and the replacements haven't come yet (I called Avent back and they said they are back ordered). In the mean time, I went back to the store and bought the 9 oz newer version with the ring (they are clear not amber) and it still didn't fix the leaking problem. They are better, but still leak every once in a while.
I'm done with Avent, I switched to Born Free bottles. The nipple collar has ridges and is easier to open when wet and more importantly, they don't leak.