Bug

A bug is basically an error in software code that results in an incorrect output or, in worse case, a complete crash of the application. For 

Some software bugs are minor in nature and may not be indentified for some time until a user reports the issue. Servere bugs, on the other hand, can had disaterous consquesces including crashing of the platform or simple inablility for the user to complete a core task.

A 'security bug' is another type of software bug which can result in unauthorised access to data or source code. 

Finding, re-creating and fixing bugs is an ogoing process for most software development to ensure the system remains stable for users.  

The majority of bugs are caused by human error, in either source code or frontend design.