How can I tell if an Illumina run is gone bad?

[PFS]

Sorry for the very basic question. I am dealing with "raw" Illumina data for the first time and I am not sure how to pre-process the reads.

The *sequence.txt file for each lane contains reads that have tons of 'N' at the end, with quality 'B'. (see below)

How can I tell if this is normal or if something went wrong with the sequencing run?

For example:
@HWI-EAS413_0021:2:1:1159:4065#0/1
TCGTGCCCGGGTAGCTCTGACTGGGCTGACTGTGGCTGAATACTTNAGNGACNANGAAGGTCANGAGATCGG
+HWI-EAS413_0021:2:1:1159:4065#0/1
dddb\dddcd^aaccdcdd\cdd^dadddc^c`ccdcd`d^`b``B\ZB`UYB]BUUU^][]_B_VWWWYT^
@HWI-EAS413_0021:2:1:1159:11764#0/1
GTGTGCCTGGTCATGCTGTGGTGGATCACCGTCCCAGGGCATTGGNGANTTNNGNATTTACGANATCGGAAG
+HWI-EAS413_0021:2:1:1159:11764#0/1
fffefffffffffeffffdff_ffefffffffeff`eedd`a^b`B\VB`]BB]B_aa\^_`_BaO[[Y]dc

[dawe]

GAP >= 1.5 (or OLB...) call bases in a slightly different way, so that if you have a bad quality read, instead of having low scores on each base, you'll see a series of at least 3 consecutive 'B' (or '#' if you converted in sanger) in the "bad zone" (but you'd better check the manual about the number of consecutives). If you see a number of reads mostly called with 'B' qual, you may have an issue. The reads you posted look good.
This is an example of a bad run:

@ILLUMINA-F3E58E_10:5:1:2798:3037
ATGGGCCCTTTATTTCCCATCTTTTCCCAACTTTTT
+
A@=>@###############################
@ILLUMINA-F3E58E_10:5:1:2798:15617
CATGTATCATGGTCATTTAGCATTATCAAATGTCCT
+
EDEABBB@BA:@:??B@:555:@@::5:??@,<B:A
@ILLUMINA-F3E58E_10:5:1:2798:10376
TCCCAACAGAACAGCCCCAAGGGGCAAGGGGTTAAA
+
A7@#################################
@ILLUMINA-F3E58E_10:5:1:2798:1024
TNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNN
+
####################################
@ILLUMINA-F3E58E_10:5:1:2798:1640
ATTGTTCCATCACCTCTGACTTTTTCCCCCACTTTT
+
@?##################################

[maubp]

This 'B' quality (PHRED 2) is the Illumina 1.5+ Read Segment Quality Control Indicator (RSQCI), see also:

http://seqanswers.com/forums/showpost.php?p=17491&postcount=3

http://news.open-bio.org/news/2010/04/illumina-q2-trim-fastq/