StarCraft does not perform bounds checking on the unit IDs and player IDs of preplaced units as it does with creating units via triggers, therefore the limit for these values are only defined by the byte count of them defined by StarCraft. 2 bytes are allocated for unit ID, making the limit 65535 ((2^8)^2-1), and 1 byte for player ID with the limit 255 ((2^8)^1-1). As StarCraft only allocates data structures large enough to contain the valid players/units, providing it with values above the limits StarCraft expects will make it write data ahead of the intended data structures, possibly in locations useful for map applications.
One of the many buffer overflows take that place is in unit scores. Referring to Farty1Billion's EUD DB, the Score Unit Total located at 0x00581ED4 will be used as an address base to calculate the address in which to increase the value by the unit score of the unit being created, using the following formula: 0x00581ED4 + 4 * player ID. This overflow is used particularly in Extended Supply.
Contrasted with EUD Actions having one easy to control buffer overflow, extended preplaced units are much more erratic and write data in many places difficult to rigorously document.
Unit Speed Acceleration
Probably the most used, the player 15 scarab, interceptor, and map revealer appear to speed units up in the game. From what I tested the scarab and interceptor speed up the units placed after it in terms of animation, movement speed, and reload speed, and the map revealer appears to just speed up the reload speed.
Units placed before them appear to be unusable.
An explanation for this effect is still unknown.
Extended Supply
The three values for supply (available, used, and max) for almost each race can be increased with extended preplaced units. Place units for the following players to increase each supply by a half of the unit score:
Zerg Available: Player 157
Zerg Used: Player 169
Zerg Max: Player 181
Terran Available: Player 193
Terran Used: Player 205
Terran Max: Player 217
Protoss Available: Player 229
Protoss Used: Player 241
Protoss Max: Player 253
Terrain Palette Modification (?)
Placing a player 20 scarab, interceptor, or map revealer radically modifies the terrain, and must be the first placed unit. Most likely it modifies a pointer to terrain palette data which would cause havoc in terms of color representation.
Discussion is encouraged.
None.