Actually, as it turns out, that error message is the modern equivalent of “your telephone is not compatible with the Bell System,” or something like that. I thought the problem was that the Phillies’ web site was too busy, but the problem seems to have been that their ticketing process was incompatible with Macintosh web browsers. Everything worked perfectly on my PC at work, so we now have 12th-row upper-deck seats to see the Phillies vs. Brewers at the new Citizens Bank Park.
Turns out the modern equivalent of a busy signal is the message “DTWP001E: Net.Data is unable to locate the macro file err_bad_user_init.d2w.” (I got up ridiculously early this morning and have been looking at this, online and related gibberish, for hours now.)