I'm sure it only throws errors when you try fishing twice at the same time

Err... not only then.

If you're in the mood, check that thread I pointed to.

Never happens to me

I bet you didn't do the amount of fishing I did over a very short time span (when fishing to get my points for the big 500 turns/hour upgrade) hahaha!

Also, during that time, I was using that creepy browser called Opera, which allowed me to spot these rare flaws.

lol i got this error 3 times in a row ;D

Is there any characteristics that lead up to this error?

I mean, does it happen when your connection lags?