i have noticed that on some spell like Fire V that the spell cast timer is wrong(the one on the upper left of the screen)
for Fire V you can cast again when the timer is at 60-65%
this is also the same on other spells but i dont have a complete list

Platform:PC

ISP:ATT

Type of Internet Connection: DSL

Internet Connection Speed:2Mb

Date & Time:
Date and time of when the issue occurred. Please be sure to state the time zone of your region. (Example: Pacific Time = PST/PDT)

Frequency:Always

Character Name:Kigensuro

Race:Hume M

World:Carbuncle

Main Job: BLM

Support Job: DNC

Party or Solo:Solo

Steps:
1. cast spell
2. wait for spell to hit mob
3. look at cast timer(still time left)
4. cast another spell(even tho cast timer says you cant cast)
5. holy cow the spell casts