The update's effects appear to vary depending on the machine and operating system used, but some Windows users are reporting their files as having disappeared completely.
According to the Softpedia newswire, triggering an update procedure from the anti-virus program should – in theory at least – resolve the problem, "but according to reports, things didn't go smoothly for everyone."
"That's because after seeing the warnings, some people restarted their computers and initiated a so-called boot-time scan", says the newswire.
"This scan, which takes place before Windows loads, detected infections in local html and javascript files and moved them to the virus chest (quarantine)", the newswire adds.
Softpedia notes that some users were able restore them after installing the new definition update, "but others who uninstalled Avast after the boot-time scan didn't resolve the problem, leading to their permanent loss."
Avast withdrew the update within an hour of it being pushed to users, but it is not clear how many users of the popular freeware IT security application were affected.
In a blog posting, the firm apologised for the error and claims a fix was released about 45 minutes after the problematic update, with version number 110411-2.
"Anyone who still has this problem is kindly asked to manually update the definitions to the latest version, e.g. by right-clicking the Avast taskbar icon (the orange (a) ball), and selecting Update -> Engine and Virus Definitions", says the company.
"We sincerely apologise for the inconvenience. As this typically only affected remote sites (and not local files), simply updating to the latest definitions should completely solve the issue (no local files have been quarantined)", the firm adds.
As Softpedia reports, "it's worth noting that Avast is not the only anti-virus company to have experienced a false positive. Almost all AV vendors have gone through this at one point or another."