Skype users are taking to its support forums to warn that some instant messages from one contact are being sent to an unintended recipient on that user's contact list.

It appears the bug may have stemmed from a Skype upgrade last month, according to the support thread.

"SunnyLady78" kicked off the thread by noting how "surprised" and "shocked" she was to find that messages sent to one person ended up being read by another one of her contacts.

Sometimes when I open chat window with some contact, it lists all latest messages from that person although I have already read them. Few days ago one friend from my contact list has received few messages I had already received before from other my Skype contact. It seemed these messages were sent by me. It is really confusing.

In reply, "strayle" added his or her two cents:

Messages sent by one contact (2 lines out of a hundred or so) were sent to another contact of mine. These 2 contacts are not connected on Skype.

Another user, who goes by the name "bla84" noted:

Strangely enough, the two messages were actually sent today on July 11, 2012 (Beijing time), but when I logged in to Skype this morning, the two messages appeared in the chat history with my contact, and it recorded it as being sent on July 10..... This is very disturbing and a serious breach of privacy, please address the issue immediately.

A Skype community manager named "Claudius" left a note in reply to the thread informing Skype users that the company was investigating.

Thanks for your reports and sorry for the inconvenience caused by this. We are currently investigating and hope to provide a solution for this soon.

Skype became a Microsoft division on October 13, a week after European antitrust authorities approved the buy-out following an $8.5 billion bid in early 2011.

A Skype spokesperson confirmed the bug in an emailed statement and said the firm would get to the bottom of it.

We are aware that in rare circumstances [instant messages] between two contacts could be sent to an unintended third contact. We are rolling out a fix for this issue in the next few days and will notify our users to download an updated version of Skype.

In a blog post published a few hours later, Skype explained the fault in further detail. The Microsoft division said it doesn't know how many users have been affected, but believes "the number is small."

"This issue occurs only when a user's Skype client crashes during a Skype IM session, which may in some cases result in the last IM entered or sent prior to the crash being delivered to a different IM contact after the Skype client is rebooted or logged in as a new user," Skype engineer Leonas Sendrauskas said.

The post added that that affected clients include Skype 5.9 and 5.10 for Windows, Skype 5.8 for Mac, and Skype 4.0 for Linux. Mobile users on iOS, Android, and Windows Phone may also be affected.

Updated at 3:00 p.m. PDT with new statement from Skype.