Well, yesterday I made a rather significant change to the code. Essentially, a reimplementation of an existing function in a completely different way, to avoid a memory consumption issue that cannot be solved in the old approach. 68 insertions, 15 deletions. It, unexpectedly, worked correctly and passed all tests from the first attempt, even though there are threads involved, and they are my weak point. But I didn't have a "Huh... that's pretty cool" moment. I had an "I don't like this code, it's too complex, it must be wrong somehow" moment.

or just feel both.. at the same time.

Only a small fraction of segmentation-fault-causing errors will be revealed in 2 seconds of testing. Thinking about all the ways your code could fail, and being able to keep a mental track of the constraints you need to observe, is a useful skill to acquire.