ARM crashing upon returning from a function

The device consists of an Cortex M4 main processor and an Cortex M0+ co-processor running the network stack.

I've been trying to influence the bluetooth advertisement beacon behavior - when are they transmitted and what payload do they carry. However any time I touch anything the CPU dies just before returning from a specific void function. I can reliably detect everything is OK just before the return:

• return address has not been overwritten during function execution
• no object is allocated or constructed, it's a void function
• no d-tors called, this is C
• the architecture requires 4 byte alignment: honored.

Yet just after the return a basic gpio set never takes place. Where do I look? The build system is somewhat complicated so simply sticking an -E isn't so straightforward.