Programming cautions

When you're coding, you will have to keep an eye out on (common) mistakes.

Confusing 8-bit values with 16-bit values

Don't try to load an 8-bit value into AXY when AXY is in 16-bit mode, and the other way around. For example, don't write LDA #$0000 when A is in 8-bit mode, as the third byte of this opcode will be interpreted as an opcode rather than a value.
Consequence(s): The game will most likely crash by interpreting instructions you never wrote.
Fixing the issue: Use the correct value size.

Looping conditions

When creating loops, don't make a small mistake which results in an infinite loop (a loop which doesn't exit).
Consequence(s): The game will lock up; the only way to exit is to reset the SNES.
Fixing the issue: Check at the end of the loop (often a comparison) to see why it doesn't allow the loop to exit. You might need a debugger for this.

Bank boundaries and edge cases

Make sure your code doesn't cross bank-boundaries ($XX:FFFF → $XX:0000) inside the ROM.
Consequence(s): The SNES would read bogus instructions and most likely crash.
Fixing the issue: The code should remain within a bank. If your code doesn't fit inside a bank, you should split up your code across banks and make use of the JSL and JML instructions.

Pushes and pulls

Make sure you pull the same amount of bytes as you have pushed, before a return instruction (RTS, RTL).
Consequence(s): If you don't, the SNES won't get the right return address from the stack and most likely crash.
Fixing the issue: Keep the amounts of pushes and pulls at an equilibrium. Especially keep an eye out on different A, X and Y modes (8-bit and 16-bit), as pushing in 16-bit mode means pulling twice in 8-bit mode. The other way around is also true.