It's work on L4D2, just need to add precache.
I have this error too, sometime :
Code:
[SM] Plugin "Witches.smx" encountered error 23: Native detected error
[SM] Invalid timer handle 650806c8 (error 3) during timer end, displayed function is timer callback, not the stack trace
[SM] Unable to call function "SpawnAWitch" due to above error(s).