if you close NL by Alt-f4 during a level the program closes but you get an error message:
"exception EAcessViolation in module NeoLemmix.exe at 000F8023
Read of address 00688168"
I just downloaded the latest version (...18) but I noticed this for several versions now (kept forgetting about it).
This isn't critical, so I won't release a hotfix. However, I'll fix it in the new-formats version, and if any future hotfixes for the current version do come out, I'll include the fix for this if possible.
This is fixed in the current new-formats version.