This is a classic signature of arcing. This is what arc-fault breakers detect. It happened in one of two ways:
- Improperly torqued screw.
- The spring which holds the plug blade got sloppy, probably because this is a cheapie ($1.00) receptacle
As to screw torque
EC&M Magazine made a test rig and took it to four trade shows. Their data was interesting. They tallied under-torqued, torqued within 20% of spec, and over-torqued - and whether a person was an electrician or not. Both groups turned in exactly the same scores.
it was obvious during the test that people who had never made an electrical connection before had as much chance of getting it within 20% as an electrician with decades of experience.
And given the broad tolerance (+/- 20%), it's as likely that the 25% correct were simply attributable to luck.
The presence of "backstab" holes reveals this recep to be a cheapie "Builder grade". They are made to be cheap and fast to assemble, so the builder can get to closing, collect a signature and get paid. When a maintainer has a long-term stake in the reliability of the outlet, they tend to go for "spec grade" or "industrial-grade" outlets. These are in the $2.50 to $10 range, and have better everything - including deletion of the "backstab" connection often replaced by a screw-and-clamp arrangement.
The one thing going for backstabs is you can't screw up the torque setting. But they have so many other problems they are not worth using.