I have observed that an entrance takes place in the file
utmp in a wrong registry.
A user appears in the ud file entrance and when it logs-off
the system is not updated because it's not in the right
place. This causes that the user appears as logged and he's
not (the who command reports the user in the system but the
w does not!). I have noticed this error in the versions 5.1
and 5.2 with kernel 2.0.36 and 2.0.38.
Since I have pam_limits activated, the mentioned situation
generates a problem because I have the maximum number of
log-ins set to one.

The utmdump looks like this:
[8][00008][si  ][        ][            ][Oct  2 08:42:30]
[1][20019][~~  ][runlevel][~           ][Oct  2 08:42:30]
[8][00079][l3  ][        ][            ][Oct  2 08:42:39]
[7][00377][ud  ][vaxsi   ][ttyC10      ][Oct  5 14:42:08]
            ^
            |----- ERROR -------------
[6][12908][1   ][LOGIN   ][tty1        ][Oct  6 19:57:32]
[5][12030][2   ][        ][            ][Oct  6 19:53:15]
[6][10183][3   ][LOGIN   ][tty3        ][Oct  4 20:01:44]
[6][10181][4   ][LOGIN   ][tty4        ][Oct  4 20:01:42]
[6][10179][5   ][LOGIN   ][tty5        ][Oct  4 20:01:40]
[6][02192][6   ][LOGIN   ][tty6        ][Oct  4 19:15:07]
[7][00384][s0  ][funatec ][ttyS0       ][Oct  2 08:44:38]
[7][01177][s1  ][funatec ][ttyS1       ][Oct  2 08:51:34]
[5][09512][z0  ][        ][            ][Oct  2 09:46:01]
[7][29441][z1  ][cepkay  ][ttyC1       ][Oct  7 08:17:44]
[5][31314][z2  ][        ][            ][Oct  7 08:25:44]
[5][30931][z3  ][        ][            ][Oct  7 08:24:03]
[7][30438][z4  ][milk    ][ttyC4       ][Oct  7 08:22:48]
[7][25554][z5  ][taap    ][ttyC5       ][Oct  7 07:58:10]
[5][03152][z6  ][        ][            ][Oct  5 20:21:42]
[5][31196][z7  ][        ][            ][Oct  7 08:24:49]
[5][30850][z8  ][        ][            ][Oct  7 08:23:02]
[7][24303][z9  ][copiara ][ttyC9       ][Oct  7 07:46:25]
[5][31135][za  ][        ][            ][Oct  7 08:24:42]
[7][30909][zb  ][futuro  ][ttyC11      ][Oct  7 08:24:37]
[7][23338][zc  ][estabb  ][ttyC12      ][Oct  7 07:34:16]
[7][29337][zd  ][junior  ][ttyC13      ][Oct  7 08:19:52]
[5][31367][ze  ][        ][            ][Oct  7 08:25:47]
[7][29556][zf  ][enrique ][ttyC15      ][Oct  7 08:20:48]
[7][27636][z10 ][tapro   ][ttyC16      ][Oct  7 08:10:49]
[7][25392][z11 ][alcabb  ][ttyC17      ][Oct  7 07:54:18]
[5][30703][z12 ][        ][            ][Oct  7 08:22:41]
[5][18679][z13 ][        ][            ][Oct  6 20:36:31]
[7][09183][z14 ][inteba  ][ttyC20      ][Oct  2 15:29:28]
[7][05758][z15 ][tusar   ][ttyC21      ][Oct  2 09:21:31]
[7][11056][z16 ][albb    ][ttyC22      ][Oct  5 15:36:08]
[7][17636][z17 ][cereal  ][ttyC23      ][Oct  6 17:34:35]
[8][25950][p0  ][        ][ttyp0       ][Oct  5 12:16:50]
[7][31227][p1  ][aboland ][ttyp1       ][Oct  7 08:25:15]