Date/Patch of first occurence: [RF]
Detailed Description: Whenever a skill button is pressed it is not possible to use an instant cast skill at the same time. One of the two selected skills will be used instead of casting both. When an instant cast skill is used at the same time as a skill is used in combo it will usually select the instant cast skill and break the combo at the same time. The expected behavior of instant cast skills is that they can be casted at any time when not down/knocked back/stunned etc. As an example that it is just programmed wrong and not impossible to realize see the example in the video, mercenarys can be casted at the same time with a skill, the combo does not break.
How to reproduce: Click on two skill buttons very shortly after each other, one normal and one instant skill, then only one will be casted.
You currently can use instant casts while in combo, however if the instant cast is near the skill to maintain combo, or at the same time, it will break. This is because instant casts do not count towards your combo count. If you wish to use instant casts and still maintain your combo chain you must use it prior or after your skill to maintain the combo. This issue has been deemed as not a bug.