I don't know if this is SDL_Mixer's fault or SDL_DrawRect, but:
upon stress-testing my new game engine, I found I can very easily crash the program (randomly but repeatedly) by running a high number of SDL_DrawRect calls at the same time as playing a lot of sounds using SDL_Mixer.

The situation can occur with a smaller number of sounds being played back simultaneously, but can be almost insta-crashed with a high number of sounds.

With straight texture blits and no SDL_DrawRect calls, the program will not crash.
With SDL_DrawRect calls and no SDL_Mixer calls, the program will not crash.

Only when both SDL_DrawRect and SDL_Mixer calls are made, will the program crash.

When it crashes, it either crashes in:
(a) An undefined space
(b) upon the SDL_DrawRect call
(c) in an undefined space with references to either SDL2_mixer!Mix_SetReverseStereo, or SDL2_mixer!Mix_SetVolume, or both.

When it crashes codelite (my IDE) always opens 'new' ie. The -*- C++ -*- dynamic memory management header, and the final part of the backtrace is always SDL_LogCritical. Routing SDL_Log to a file gives no output from LogCritical.

There is no multithreading in the app. For each SDL_Mixer sound playback, there is typically a 	Mix_Volume(current_channel, current_volume) or 	Mix_SetPanning(current_channel, 255 - current_pan, current_pan), with it.

There are no links whatsoever between the class doing the SDL_DrawRect call and the class doing the SDL_Mixer calls. They are completely separate (one's a quadtree, the other a sound library).

I've triple-checked both modules and sections of code, and there's no issues there. The issues only show up when both DrawRect and SDL_Mixer functions are called.

Though the crash can sometimes occur with 60-or-so sounds in playback and a smaller number of SDL_Rect calls, I can nearly insta-crash it with a high number eg. 2000 sounds + > 2000 DrawRects per frame. Removing either the sounds or the DrawRects removes the crash.