This is probably wellknown to the experts but I could not find any answer neither in my head nor in the literature: Is there a (unital) C*algebra such that its projections do not form a lattice (under the usual ordering)? Certainly, this cannot be a von Neumann algebra.
2 Answers
You can find examples of AF algebras without the lattice property in Section 2 of AF Algebras with a Lattice of Projections by Aldo J. Lazar here.

$\begingroup$ Perfect! I was just commenting that in that arxiv paper there were only examples of C^*$algebras with that property and not without :) $\endgroup$ Jan 22, 2012 at 14:10
Here is perhaps the simplest example. Let $A$ be the C*algebra of all sequences of $2 \times 2$ matrices converging to a scalar multiple of diag(1,0). Let $p$ be the constant sequence diag(1,0), and $q$ a sequence of rank 1 projections converging to diag(1,0) but never exactly equal. Then $p$ and $q$ have no upper bound at all. This example can be tweaked to make it unital by allowing any limit matrix at infinity and taking $q$ to alternate diag(1,0) and nearby but unequal projections. Then $p$ and $q$ have no least upper bound.
$C^*$
algebras; i.e., $C(K)$ spaces? $\endgroup$