Hot for Typo

Many folks are fond of critiquing public signage; some even take vigilante action (as reported by blog Laughing Squid). Here's one from the Boston Public Library's special reserves desk near the sixty-minute Web room:

Check back in 30 minutes if your pager doesn't flashes.

Pretty advanced verb agreement stuff going on here. The present tense, third person auxiliary verb, with negation, doesn't, is correctly paired with the subject, your pager [it]. But flashes is conjugated to match the third person too: it flashes. Huh?

Here's what Latent Print pulls from trusty ol' MW's Collegiate (spend more time with your dictionary, k?): In the auxiliary form, do is "used with the infinitive without to to form present and past tenses ... in negative sentences." If all that grammar gibber sounds as ridiculous to you as the above sentence, you're probably fine just trusting what sounds right to you: Check back in 30 minutes if your pager doesn't [to] flash.

Beware, however, that a simple fix could throw the author's meaning even further from the mark. What if the author meant to direct you to check back if the pager flashes? What if it's a rogue doesn't? Oh my.

How do these flubs get printed? Latent Print readers, I leave the imaginary journey of that sentence to you, in comments.