Made another quick test and got additional information.
I opened the file, created a new layer and deleted all original content including objects, layers, and swatches. Created a box with a pattern from one of the Illustrator's pattern libraries. Copy paste to another document with different artboard dimension worked fine - no problem.
Repeated the same except this time I scaled up the pattern and this made the pattern shift after copy paste. Again can't reproduce this problem with any other file. So, my finding is that the corruption is limited to the file and the fact that the pattern was scaled.
Another thing that may be worth watching for is the normal Illustrator's behavior that after creating or selecting a pattern with transformations (scaled, rotated, etc.) all new objects with the same or different patterns will be created with the same transformations until the fill is not a pattern.
My nose says: This is a sneaky bug that does only occur under very specific circumstances.
Reminds me a bit of the radial gradient shifting (disappearing) bug.
Thanks emil for all the info. i think going forward i will resize all my patterns before filling...and i'll see if this shifting from occurring.
i appreciate how much investigating you did for me!
i think you may be right. i'm going to take all the suggestions given into consideration when i make my next file...and i'll see if it keep happening. i've only been using illustrator for a year or so...and i use it for very specific needs (my flat drawings and boards used in the fashion industry). so i may not have learned to make images that are flexible enough to use in many ways. i may need to re-learn making my images in the "correct" way. i'm impressed with how much everyone seems to know about the program. i'm curious to know what you use illustrator for and how long you'be been using it?