Quick answer: Crashes when the player alt-tabs out of (or back into) the game are caused by mishandling the focus-change. The classic cause is a lost graphics device in exclusive fullscreen, alt-tabbing can invalidate GPU resources, and code that uses them afterward crashes. Other causes are unsafe pause/suspend logic on focus loss. The fix is to handle device-lost and focus events properly, recreating graphics resources when focus returns.
Alt-tab crashes are a classic, the game runs fine until the player switches away and back, then it crashes. This is a focus-handling bug, and it's especially common with fullscreen games and graphics resources. It's worth fixing because alt-tabbing is something players do constantly, so an alt-tab crash hits a lot of players in normal use.
Why Alt-Tab Crashes Happen
When a player alt-tabs, the OS changes the game's focus and window state, and the game has to handle that transition. The most common crash cause is a lost graphics device: in exclusive fullscreen especially, alt-tabbing away can cause the graphics device/context to be lost or invalidated, and if the game keeps using the now-invalid GPU resources (or doesn't recreate them on return), it crashes. Modern APIs and windowed/borderless modes mitigate this, but it remains a frequent cause.
Other causes: pause/suspend logic that runs on focus loss touching state unsafely, audio or input systems mishandling the focus change, and threads or timers behaving badly while the game is backgrounded. The crash may happen on alt-tab-out (handling the focus loss) or on alt-tab-back (restoring, where lost resources are used or recreated incorrectly).
How to Diagnose It
The reproduction is usually easy, alt-tab out and back, possibly in fullscreen, possibly repeatedly, so you can often trigger it yourself. The stack trace tells you what's failing: a trace in graphics code points at the lost device / invalid resource; a trace in your pause/focus-handling code points at the focus logic. If it only happens for some players, capture it with device context, lost-device crashes can be GPU/driver-specific.
Bugnet captures the trace and device context for crashes and groups them, so if alt-tab crashes hit players (especially on certain GPUs in fullscreen), they cluster with the evidence. But because alt-tab crashes are often reproducible locally, the main value is confirming the cause and checking whether it's hardware-specific.
How to Fix It
For the lost-graphics-device case: handle the device-lost / focus-change events your graphics API provides, stop using invalidated resources, and recreate the necessary graphics resources when focus is regained, rather than assuming they survive the alt-tab. Using borderless-windowed instead of exclusive fullscreen avoids exclusive-mode device loss entirely and is a common, robust fix. For pause/focus logic, make the focus-loss and focus-regain handlers safe, don't touch or assume state that the transition invalidates, and test the full out-and-back cycle.
Test thoroughly: alt-tab out and back repeatedly, in both fullscreen and windowed modes, since the bug often only triggers in a specific mode or after multiple cycles. After fixing, verify alt-tab crashes stop in the field. Because players alt-tab constantly, a robust focus-handling implementation removes a crash that would otherwise hit a large fraction of your players during ordinary play.
Alt-tab crashes are focus-handling bugs, classically a lost graphics device in fullscreen. Handle device-lost events and recreate resources on return, or use borderless windowed.