I have been facing this issue with sqlplus for quite sometime and have been ignoring it with shortage of time as an excuse. But i couldn’t avoid any further given that i had to do lot of DB stuff recently and i needed it running fine.
Issue:
I have TOAD, SQL Developer and SQLPlus all installed. I was able to connect to my db using TOAD and SQL Developer without issues but whenever i tried connecting using SQLPlus with same db/schema/pwd it threw up following error message,
ORA-12154: TNS:could not resolve the connect identifier specified
Strange!!!. There aint any issue with tnsnames.ora or sqlnet.ora since it is being used by other tools and they work fine. I googled around and did checks here n there but no help.
Resolution:
I happened to read somewhere that some special characters like @ etc. have special meaning for sqlplus and using them in userid/password confuses it. I had an @ in my password and that was the reason for this error. I changed my pwd and it worked like charm. 
Hey Amit! Thanks for sharing this. I never knew about this. Now, this is added to my knowledgebase.
Do share such experiences with technology
Holy crap… this solved our problem after we wasted three hours on it! It was staring at us the whole time. Thanks!
Thanks!
You dont know how much time I wasted on this.