Warning! Wizards can be quite prone to experimenting spells. Now, while currently in the Hall of Attainment, the creators may be classed as mages, there is the high chance that in truth these mages could have actually been wizards.

When it comes to creating spells, it can take a lot of time and research into it, so you can see why it would be more likely a wizard to have created some of these spells despite what they call themselves in terms of types of magic practitioner.

Some examples of experimental spells - of which should note are highly dangerous to do, and would not recommend doing them - include:

Inextinguishable Fire
Perhaps to be one of the first ever experimental spells to be attempted from as far back as 3900 OE. There is no record of who created such spell, but I find it makes more sense to be from a wizard.
It is said a male elf had tried to cast this spell to be able to engulf his sword in a never-ending flame. He continued to increase the intensity and the temperature but soon lost control.

Infinite Mass Attacks
Was created by Krenic Aeden, a male elf in 1450 OE. He joined a guild that was dedicated to fighting and organising fights amongst the members. Though as a participant he started growing bitter towards those he had lost against, and so wished to develop his skills with magic.

He came up with a spell through research that would increase his strength infinitely and allow him to make the so-called Infinite Mass Attacks that would essentially allow him to beat any opponent with a single hit. But when it came to actually using the spell during a fight, it went sideways very quickly; his body couldn’t handle the stress and started getting harder and denser, before eventually breaking down and crumbling away.

~ The Mystic, Hecate

For more detailed sources of information, see the following links:
Wikipedia: Magician
Types of Magic Practitioners
Experimental Spells