Windows Version (but I don't believe it matters)

When casting an ability effect on an ally or enemy at the exact time as the effect fades off that ally or enemy, the effect will say No Effect and not apply causing you to have to recast it.

This seems to affect all abilities ranging from various DoTs to buffs and heals like Regen. I have personally noticed it most frequently happening on white mage debuffs / regen / dots because I play a white mage.

This is most easily seen when trying to spam Holy. The first holy stun will go off, and the second stun happens (for my cast time of holy wearing full darklight and a standard white mage relic weapon) to be timed exactly such that if you try just spamming holy it will say No Effect always for the second stun, but if you wait a half-second before casting the second Holy the second stun will apply properly.

This bug can definitely be detrimental if you are unlucky enough to have an important cast not apply, and affects the DPS of anyone who uses DoTs and accidentally re-applies them too perfectly, and for me is a reason why I wouldn't choose a DoT based caster (summoner).