Some other things I've noticed:
5] When bumping the paddle with the left mouse button, it will remain in a hovering state if the button is held. This can be kind of annoying, as it increases the risk of slicing a ball underneath the paddle.
6] When pausing the game, the only way to close the game options dialogue box is by clicking the Continue button. This means that that the cursor has to be moved before returning to the game, thus increasing the risk of missing a catch. It would be more convenient if the game could be continued by pressing one of the keys again (or Enter). The same thing also applies with the tutorial messages.
7] The fireball has a slightly visible frame surrounding it, which gives it the appearance of a square on brighter backgrounds.
8] The star bell on the fifth board of the Freeware set does not respond to fireballs. It also has a slow response rate, as it will not trigger if hit rapidly (might want to disable its animation for the miniguns).
9] When proceeding from the Select Player screen to the Select Arena screen, the Select Player banner on top of the screen is briefly overlapped by an alternative image during the screen transition animation.
10] The sound that plays when moving sliders appears to be re-triggered per step on the slider, creating a messy rendition with clicks and pops. Also, the sound will continue to loop forever if the cursor is dragged and dropped over the OK or Cancel buttons.
12] If quitting a game or finishing a round while using the flamethrower, the game will play the sound of unloading the weapon when the screen transits. The same applies with the miniguns, and if quitting a game while simultaneously launching a ball.
13] When adjusting the volume of the sound from the in-game options menu, the preview sound effect fades out instead of looping. Also, when adjusting the volume at lower levels, the preview sound becomes more click-like.
14] The game credits display an outdated version number.
15] Icons are missing for 'Game connection: Good' and 'Game connection: Poor' in the tooltip that appears when hovering over the icons of a hosted game online.
16a] The sound effect for an impact between the paddle and the ball also has a slow response rate. In other words, if two balls hit the paddle rapidly, only the first impact will trigger the sound effect.
16b] Balls do not make any sound upon impact with the opponent's paddle, unless the opponent punches the ball.
17] The score notifier, that appears and fades when scoring points, is not summed when the player catches a power-up with one or both of the score multipliers active. Instead it will display 20 × 2, 20 × 5, or 20 × 10, whereas brick impacts are fully summed, e.g. 40 × 2 or 100 × 5.
18] Many of the letters that make out a player's name at the beginning of a match are vertically misaligned, making the names appear warped.
19] When a paddle power-up replaces another, like the flamethrower being caught while still having ammo left in the miniguns, the icon for the previous power-up will still remain active on the status bar.
20] The red and blue banners are not swapped when the local player is red. From a logical perspective, it would seem more reasonable if the opponent's name is is always listed on the top, and the local player on the bottom. This also applies for the news scroller, which is always blue for the local player.
21] On board 9 in the Stone Mosaic set, there is no sound effect upon impact with the narrow white towers.
22] Fireballs always overlap obstacles that have a behind, e.g. white towers, as opposed to regular balls.