Dozy,
Clandestino is spot-on: Strasbourg and Bangalore. (Right now, I can't remember which came first).
Strasbourg
resulted in a mod to the FCU, and the later addition of the selected rate to the relevant FMA on the PFD (VS or FPA). Because of limitations of space, the relevant FCU selector knob and associated selected-rate window are dual-function: VS or FPA. Changing from VS (actually HDG/VS) mode to FPA (actually TRK/FPA) mode is achieved by a push-button, confirmed by an adjacent indicator. Originally, IIRC, a selected VS of 3300ft/min DOWN would be displayed in the selected-rate window as -33. A selected FPA of 3.3 deg DOWN would be displayed as -3.3, IIRC. The similarity of the two readings is thought to have misled the crew but, as Clandestino also points out, that assumes the crew also failed to ensure that the relevant FMA was indicating FPA (if the theory is correct, it would have been indicating VS). The FCU modification changed the selected VS displayed from hundreds of feet/min to ft/min, giving -3300 in the above case. Once the FMA addition was incorporated, we encouraged crews to use that as the primary read-out, once you had found the knob and started turning it. That way, you observe the changing rate on the PFD, and minimise interruption of your flight-instrument scan.
The following is off-topic, and will be old-hat to pilots of aircraft with full EFIS, but may be valuable news to others:Operating Airbuses (and other modern types) demands the constant use and monitoring of the FMAs (flight-mode annunciators). Incorporated into the PFD, they enable you to confirm you have the desired autoflight mode, while minimising the time your scan is removed from the PFD. (The same applies to the indices of selected speed, selected HDG/TRK, and selected altitude.)
Some FMA changes are worthy of mandatory callouts, but these are all worthless unless made by the pilot NOT operating the FCU. (When the AP is engaged, FCU selections are normally made by the PF.) Unfortunately, no two fleets agree which calls should be made (calling them all would be bedlam). Where available, FMAs are the primary indicators of autoflight configuration, with or without the AP.
Moving on to Bangalore, the same applies. For the descent, the practice of dialling zero feet into the altitude selector and selecting OP DES ("open descent") with A/THR was always a hostage to fortune, but where it became insidiously dangerous was when the PF elected to break off instruments, and fly a visual approach without turning off both flight directors (FD1 and FD2). The reason is that OP DES forces the A/THR into idle mode, which is clearly indicated as IDLE on the thrust FMA. The FD gives pitch-attitude commands to maintain the speed selected by the crew (and displayed on the ASI). If and when the PF ceases to follow those commands, the speed will rise or fall, the thrust staying at idle (unless manual thrust is used). We quickly learned to turn off both FDs, and monitor the thrust FMA change to SPD (speed mode), in which the autothrust controls the speed. Sadly, that lesson had apparently not been learned by the crew at Bangalore. (Note my paragraph above.) After the accident, Airbus introduced an automatic mode change of the A/THR from IDLE mode to SPD mode in the event that the current speed falls below (IIRC) VLS.