Limit on number of groups a user can join

[Andrea Venturoli]

Today I added an user to a new group and suddenly he reported he could 
not use this server anymore: he could not login via ssh or access Samba 
shares.
I tried (as root) "su username" and it failed.
I removed it from that group and everything worked fine again.
I made some tries removing him from other groups and I got to the 
conclusion that it works as long as he is in no more than 15 groups, but 
breaks when he join the 16th.
Is this an hard limit? Can it be extended? Why this?

  bye & Thanks
	av.