Metabolic slowing is offensive except against trusting targets. Since you trust yourself it's not offensive when cast in that regard, anyone not trusting will attack you for casting it.

Dispel is always offensive and the helpfile says it's considered an attack. (I don't know where it says casting/communing attacks on yourself lowers %, but it's probably meant to eliminate people spamming on themself to master things.)
Cancellation is the non-offensive equivalent of dispel, but can only be cast/communed on trusting people.

I had that happen on an invoker I played, where I cast a spell at myself accidentally because I had a name similar to my target and lost skill %. I only noticed because I started learning it again after thinking I had perfected it to know I had lost the %.