Tasks are saved in filesystem AND registry

Tasks are stored in 3 locations: 1 file system location and 2 registry locations.

File system:

C:\Windows\System32\Tasks

Registry:

HKLM\Software\Microsoft\Windows NT\CurrentVersion\Schedule\Taskcache\TasksHKLM\Software\Microsoft\Windows NT\CurrentVersion\Schedule\Taskcache\Tree

So, you need to delete a corrupted task in these 3 locations.

I want to extend @Jan answer:

It's seems, that Task Scheduler 1.0 API uses C:\Windows\Tasks folder for create and enumerate tasks (this example), while Task Scheduler 2.0 API uses C:\Windows\System32\Tasks to create and enumerate tasks (this example).

It's also seems, that windows console utility schtasks and GUI utility taskschd.msc uses Task Scheduler 2.0 API.

P.S.I found, that if task placed in C:\Windows\Tasks and have not set AccountInformation, then task won't be displayed in windows console and GUI schedulers. If you set AccountInformation (even "" for SYSTEM account) and set flag TASK_FLAG_RUN_ONLY_IF_LOGGED_ON - task will be displayed in all standard applications.

Solution found here

For Windows 7 and up, scheduled tasks are not run by cmd.exe, but rather by MMC (Microsoft Management Console). %SystemRoot%\Tasks should work on any other Windows version though.