Pitfall18 - thread safe

when you use a seperate thread to save a file,
in the thread you define the file name as a variable "FileName"
in the main program, you use variable "FileName" to check if file exists and if yes, you continue to use the file name
if you are in a loop

it is possible that: 
round 1 you save file-1 and use file-1, everythin is ok
round 2 you save file-2 but you still use file-1, what happen?
at the time when you use variable "FileName" check if file exists, you still check with file-1 (because file-2 is not saved yet and more importantly the variable "FileName" is still pointing to file-1), 
and it does exist and then you proceed with using file-1 again!

summary:
in your main program, you use a variable defined and being changed in a thread
if update on the variable is delayed somehow in the thread, meanwhile in your main program which is still running, when you access to the variable, it is not yet updated.