Vendor provided an outlook plugin (ew) that linked storage directly in outlook (double ew) and contained a built in pdf viewer (disgusting) for law firms to manage their cases.
One user, regardless of PC, user account or any other isolation factor, would reliably crash the program and outlook with it.
She could work for 40 minutes on another users logged in account on another PC and reproduce the issue.
Turns out it was a memory allocation issue. When you open a file saved in the addons storage, via the built in pdf viewer, it would allocate memory for it. However, when you close the pdf file, it would not deallocate that memory. After debugging her usage for some time, I noted that there was a memory deallocation, but it was performed at intervals.
If there were 20 or so pdf allocations and then she switched customer case file before a deallocation, regardless of available memory, the memory allocation system in the addon would shit the bed and crash.
This one user, an absolute powerhouse of a woman I must say, could type 300 wpm and would rapidly read -> close -> assign -> allocate -> write notes faster than anyone I have ever seen before. We legitimately got her to rate limit herself to 2 files per 10 minutes as an initial workaround while waiting for a patch from the vendor.
I had to write one hell of a bug report to the vendor before they would even look at it. Naturally they could not reproduce the error through their normal tests and tried closing the bug on me several times. The first update they rolled out upped it to something like 40 pdfs viewed every 15 minutes. But she still managed to touch the new ceiling on occasion (I imagine billing each of those customers 7 minutes a pop or whatever law firms do) and ultimately they had to rewrite the entire memory system.