The sprites I placed mysteriously disappear. How do I fix this?
It's likely you have either encountered an item memory glitch, or reached the sprite limit.
Sprite limits are simple: if too many sprites are displayed on the screen at the same time, new sprites will not appear. Also, the game generally slows down when there are a large number of sprites on-screen.
An alternate type of sprite limit can occur when you're using too many different types of Super FX sprites (these are generally sprites that rotate/stretch in some manner) in a single room. If your goal ring vanishes and there aren't other sprites around, Super FX sprites might be the issue. Try removing one enemy/obstacle type in that level, or add a screen exit to a separate room. Super FX sprites only take up memory if they've appeared since the last screen exit you entered. Also, if your sprite set supports S110 flower (which does not use Super FX), you can use that instead of SFA flower (which does use Super FX).
The items I placed mysteriously disappear. How do I fix this?
If you place any collectable item directly above/below another collectable item, in the same screen, collecting one item will cause the other(s) to vanish if you scroll it off-screen and back on.
Collectable items include:
Coins (including ! switch coins)
Red coins (including Fly Guys and Bandits with red coins)
Fly Guys of any type
Clouds (only some types, such as the ones with stars, flowers, 1ups, ! switches, or morph bubbles)
Locked doors (the lock is the "collectable" item, since it vanishes permanently)
S161, the bonus sprite (it's always affected by item memory, even if it's a door)
They do not include:
? clouds that respawn after reloading the level (if it contains a red bridge/staircase or a watermelon for example)
Note that normal coins will not vanish until you actually reload the area (via a screen exit). If you place only normal coins above/below each other, item memory isn't really an issue. What matters is if a major item vanishes.
I didn't place any items directly above/below each other, so why do my items still vanish?
Each level contains an item memory/Midring header setting. If two or more levels share the same setting, collecting items in one may cause items in another to vanish.
It is highly recommended that you make every sublevel in your level use a different item memory setting. For example, if 4-4 contains Levels 1E, 55, AA, and C1, set their Item Memory Indexes to 0, 1, 2, and 3, in any order.
Why do some sprites/my level's goal appear glitched after a Chomp Shark chase?
For whatever reason, after sprite 154 is defeated and falls off the screen, the available graphics space for SuperFX sprites is reduced for the rest of that room, making them share graphics. The most common place to see this issue is if you have the goal ring immediately after a Chomp Shark chase. If you wait for it to fall off the screen, the goal graphics will become corrupted.
An easy fix is to simply avoid having more than one SuperFX sprite on the screen at any time in sections that use this sprite. In the case of goals, make sure there is a screen exit separating the goal area from the chase area.
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