No CUPS server setup can be considered complete without sending a print job to the wrong printer:
Which wouldn’t be quite so bad if the printer weren’t ever so much faster than I am:
It turns out an ordinary clothes iron can flatten those pages. Set it to “silk”, spread packing paper on the ironing board to intercept the toner, iron a few millimeters of pages at a time, and feed them back into the printer.
Back in the day, laser-specific printer paper came with a grain arranged so it wouldn’t curl when you fed it into the printer with the proper side up. Those days are gone; I’ve tried both ways and they both curl.
Protip: When CUPS thinks it’s done with the job and the Web interface shows nothing’s going on, it’s handed the job to the server’s printing subsystem, which continues spooling data to the printer. Choking off the bitstream requires one command-line invocation on the server connected to the printer:
cancel -a
A paper jam gives you enough time to figure all that out.
The Smell of Molten Projects in the Morning 
While curl has a lot to do with grain, it’s mostly down to moisture content. A trip through a laser printer dries out paper quite a bit, sometimes more on one side than the other. A few days at ambient temperature can help get them flat again.
Big old laser printers (I’m thinking particularly of the HP LaserJet 4Si) heated paper so much that it would be measurably smaller when it came out. For producing fastidiously-correct page proofs for editorial signoff we’d have to correct for this shrinkage in a variety of time-consuming ways
The Basement Laboratory hovers around around 55% RH during the summer and I don’t recall quite so much curling, but the end of the dehumidification season has arrived with 65% RH: evidently enough water to curl my pages. Of course, if I weren’t such a cheapskate, I’d just recycle the pages and move on, but they’re so empty …