Amazon.com encountered a firestorm today over gay- and lesbian-themed books which apparently had their sales rank removed from the retailer's website. The situation sparked a barrage of criticism online, on Twitter and elsewhere. An Amazon spokeswoman blamed the problem on a "glitch" and said it is being fixed.

Author Mark Probst noted the sales rank removal on his blog and wrote that, "Many of us decided to write to Amazon questioning why our rankings had disappeared. Most received evasive replies from customer service reps not versed in what was happening." He said he later got a response from Amazon explaining that "we exclude 'adult' material from appearing in some searches and best seller lists. Since these lists are generated using sales ranks, adult materials must also be excluded from that feature."

That sales rank is a critical feature to authors. As Cnet notes, "being delisted from the rankings doesn't mean that the book giant has stopped selling the title; it just means that the title won't show up with a public sales ranking or in the best-seller lists -- often a factor in how shoppers make their purchases."

"There was a glitch in our systems and it's being fixed," Amazon spokeswoman Patty Smith said. Asked for an update Sunday night, she said: "I don't have anything to add at this time."

The sales rank issue has ignited a storm on Twitter under the tag #amazonfail.

Update: The Gay and Lesbian Alliance Against Defamation (GLAAD) said it expects Amazon's "error" to be corrected immediately. Here's the full statement:

"GLAAD has reached out to Amazon.com and they indicate this was an error, so we expect to start seeing evidence of its correction immediately, and that any loss of visibility of gay-themed books as a result of this error will be made right by Amazon," said Neil G. Giuliano, President of the Gay & Lesbian Alliance Against Defamation (GLAAD). "When people learn about the lives of gay and transgender people and the common ground we share, the culture changes and advances. It is so important that stories about the lives of our community are available, and that companies like Amazon promote these titles in an equal fashion."

Update: Amazon called the sales rank issue a "embarrassing and ham-fisted cataloging error" and says it goes beyond gay titles.