Mac Classic System Error 28 — Stack Ran Into Heap

What Does This Error Mean?

System error 28 means the program stack expanded into the heap area — they collided.
Usually a sign of deep recursion, a runaway program, or insufficient memory for the running app.
Restart, then either increase the app's memory partition in Get Info or stop running the program that crashed.
Sometimes points to a bug in the app itself, not your fault.

Affected Models

Macintosh Classic
Macintosh Classic II
Macintosh SE
Macintosh SE/30
Macintosh II
Macintosh LC
Macintosh Plus

Common Causes

Stack overflow from deep or infinite recursion
App memory partition too small
Buggy software with a memory bug
System extensions using too much stack space

How to Fix It

Restart the Mac.

Force-restart with the programmer's switch or Cmd-Ctrl-Power.
Don't try to keep using the system after error 28 — heap corruption can spread.
Boot with extensions off.

Hold Shift while restarting.
If the app runs cleanly without extensions, an INIT was eating stack.
Re-enable extensions one at a time using Extensions Manager to find the culprit.
Increase the app's memory partition.

Single-click the app icon, File → Get Info.
Raise both Preferred and Minimum sizes.
Try going from 512K to 1024K, or whatever doubles the current value.
This gives the heap more room and pushes the stack further away.
Update the application.

Older versions of some apps had stack-growth bugs.
Check if a later version exists.
Many classic Mac apps had stack bugs fixed across minor revisions.

Frequently Asked Questions

Stack and heap — which is which on a Mac?

On 68k Mac OS, each application has a memory partition.
The heap grows upward from the bottom; the stack grows downward from the top.
If they meet, error 28.
Bigger partition = more room for both.