It is also possible to put in the shotgun into the flaregun or a magazine.. or anything else I believe, here is example of glitches that can be produced with that bug (video is not mine):
https://www.youtube.com/watch?v=M3ht9DLTokA
This needs to be fixed ASAP.

I confirm this issue. I have read about this issue on various forums and it seems to have been in the game before in around 2014 but I never had (have been playing since 0.28) until 0.55 update.
Whenever I try to play now (0.55) I always end up stumbling upon invisible zombie (even if I try to use new stealth mechanics) that attacks me and takes normal zombie damage. I have been forced to stop playing until this is fixed as it is impossible for me to play/test game without being constantly hit by invisible and silent zombie and dying. Lets wait until 0.56 zombies should be more stable by then.

This has started to happen frequently to me after 0.52 aka Xmass update - before I had this issue as well but exiting the server as soon as I saw the shore and no gear and then logging in on another server always fixed the issue - this time it sometimes works sometimes not - but in 4 consecutive games (4 days in a row)I had this problem three times and once I was able to save my character (that did not have much more than chicken feathers anyway) and twice I lost my character. I stopped caring about how much gear I gathered as I always loose it that way (some bug e.g. trampoline rocks) but this issue is really serious - If you can't trust your gear will be there next time you log in, then what is the point of playing? You won't even test the game mechanics as you will have to start all over again instead of trying new things added (e.g. ghillie suit, trucks, new weapons).

I can confirm I experience the same.
Used to sometimes happen before but just exiting and relogging always worked. After 0.52 update it doesn't help.
This is also the same as: http://feedback.dayzgame.com/view.php?id=20083

0.48 This also happens when you hold two-handed weapon (I had Mosin with all attachments etc.) and search for kindlings. And yes.. it looks pretty gangsta but it's glitch that doesnt make shooting easier.