Many people do not, or do not consistently, distinguish in pronunciation between instance and incidence.
An incident is “an event, especially one that is unusual, important, or violent” (LDOCE); incidence [usually singular] is “the number of times something happens, especially crime, disease etc”. Despite these dictionary definitions, a quick Google search throws up 35,000 instances (yes!) of isolated incidences.
I wrote about this on 9 May 2008. Here’s what I said then.
The confusion arises because instance and incidence may be pronounced identically in rapid speech. This is because of the possible disappearance of the vowel of the middle syllable of incidence. We can, and often do, go straight from the s to the d, omitting the weak vowel (ə, or a conservative ɪ) that would otherwise stand between them. Since under these circumstances the d, now abutting on a voiceless consonant, gets devoiced, the result is that the -sd̥- of incidence ends up very similar to the -st- of instance.
In moderately-paced speech the deleted vowel seems to leave some compensatory lengthening of the preceding consonant: ˈɪn(t)sːd̥ən(t)s.
In rapid speech, however, I think this subtlety of timing can be lost, making incidence as good as homophonous with instance.
At UCL some of us at one time called this phenomenon ‘pseudo-elision’, as opposed to true elision where the deleted segment supposedly leaves no trace at all.
We find the same thing in words such as trinity, comedy, Cassidy, quality, university (can it rhyme with thirsty?). In trinity the tongue tip may remain in place on the alveolar ridge as we pass from the (lengthened?) [n] to the [t], with no intervening vowel.
At the time I wrote that, dear readers, you could not comment on it. Now (if you wish) you can.