Routine peripheral blood smears can rarely show clusters of endothelial cells (1). This almost always occur as a result of poor procedural technique. This can be caused when using large bore needles, or, using a needle with a blunted tip from repeated unsuccessful atte venipuncture attempts. Historically, multi-use needles and non-beveled needle-tips were major causes of these incidental findings (2).  

Inability to recognize these cells correctly is a potential diagnostic pitfall. It may lead to misinterpretation as a significant abnormality and trigger unnecessary additional diagnostic workup. 

Although, the presence of endothelial cells is mostly iatrogenic, circulating single endothelial cells are a normal finding. The literature has many reports that detected and enumerated endothelial cells in the buffy coat collected from blood samples after centrifugation. Light microscopy and flowcytometry studies were used for characterization. Many correlations between the percentage and activation status of circulating endothelial cells and disease state exists. Higher numbers of circulating endothelial cells are seen in association with many inflammatory and immune mediated conditions including vasculitides (3) and infections (4). 

References: 
1. Cha C-H, Kim J. Endothelial cells in peripheral blood smear: an artifact? Korean J Hematology 2010;45(3):150–150.  

2. Shanberge J. Accidental Occurrence of Endothelial Cells in Peripheral Blood Smears. Am J Clin Pathol 1955;25(4_ts):460–4.  

3. Woywodt A, Streiber F, de Groot K, Regelsberger H, Haller H, Haubitz M. Circulating endothelial cells as markers for ANCA-associated small-vessel vasculitis. Lancet 2003;361(9353):206–10.  

4. Erdbruegger U, Haubitz M, Woywodt A. Circulating endothelial cells: A novel marker of endothelial damage. Clin Chim Acta 2006;373(1-2):17–26.